直す vs. 治す: Which Japanese “Naosu” Should You Use?

Introduction

Imagine hearing these two sentences in a conversation:

「パソコンをなおす。」
“I’ll fix the computer.”

「病気をなおす。」
“I’ll cure the illness.”

In spoken Japanese, both words are pronounced 「なおす」 (naosu). However, when they are written, different kanji are normally used:

• 直す (naosu / to fix, correct, or put right)
• 治す (naosu / to cure or heal)

Both words describe changing something from a bad condition to a better condition. The important difference is what is being improved.

Use 「直す」 for objects, mistakes, writing, posture, behavior, and other things that need to be corrected or restored.

Use 「治す」 for illnesses, injuries, cavities, and other health-related problems.

Rather than memorizing only dictionary definitions, it is much easier to learn these words through real situations. Let’s look at how Japanese speakers choose between them in everyday conversation, formal writing, and JLPT reading passages.

Start with a Real-Life Situation

Imagine that you are getting ready to leave for school.

Your bicycle brakes are broken, and your younger brother is lying in bed with a cold.

In this situation, you would say:

「父が自転車を直しています。」
My father is fixing the bicycle.

「弟は家で風邪を治しています。」
My younger brother is staying home and treating his cold.

A bicycle is a broken object, so 「直す」 is the natural choice.

A cold is an illness, so 「治す」 is used.

The action is similar in both cases: something in a bad condition is being made better. However, the object of the action determines which kanji is appropriate.

Situation 1: Using Them in Everyday Conversation

Both 「直す」 and 「治す」 are common in conversation. However, 「直す」 can be used in a wider range of situations.

Fixing a Broken Object

「この時計、動かなくなったんだ。」
“This watch has stopped working.”

「駅前の店で直してもらったら？」
“Why don’t you have it fixed at the shop near the station?”

When a watch, smartphone, bicycle, computer, car, or machine is broken, use 「直す」.

You may also hear 「修理する」 (shūri suru / to repair). However, 「直す」 is usually softer, simpler, and more natural in everyday conversation.

「壊れたスマートフォンを直してもらいました。」
I had my broken smartphone fixed.

「修理する」 sounds slightly more technical or formal, while 「直す」 works naturally in most daily situations.

Recovering from an Illness

「まだ風邪が治っていないの？」
“Has your cold still not gone away?”

「うん。週末はゆっくり休んで治すよ。」
“No. I’m going to rest over the weekend and get better.”

Illnesses and health problems normally take 「治す」.

It is also important to distinguish between 「治す」 and 「治る」.

• 治す (naosu / to cure something)
• 治る (naoru / to recover or be cured)

Compare:

「薬を飲んで風邪を治す。」
I will take medicine and cure my cold.

「風邪が治るまで家で休む。」
I will rest at home until my cold gets better.

「治す」 is a transitive verb. Someone actively treats or cures something.

「治る」 is an intransitive verb. The illness or injury recovers.

This transitive and intransitive verb distinction is frequently tested on the JLPT.

Correcting a Mistake

「この漢字、間違っているよ。」
“This kanji is wrong.”

「本当だ。すぐに直すね。」
“You’re right. I’ll correct it now.”

Use 「直す」 when correcting kanji, grammar, an answer, a calculation, or information in a document.

「作文の間違いを直しました。」
I corrected the mistakes in my composition.

A mistake is not a medical condition, so 「治す」 cannot normally be used here.

Situation 2: Using Them in Writing and News Reports

In conversation, 「直す」 and 「治す」 sound exactly the same. In written Japanese, however, the difference becomes clear.

Understanding the kanji can help you quickly identify what kind of situation the writer is describing.

School and Workplace Writing

At school or work, 「直す」 is often used for reports, documents, machines, and incorrect information.

「先生に言われた部分を直して、レポートを提出した。」
I corrected the sections my teacher pointed out and submitted the report.

「書類の入力ミスを直してください。」
Please correct the data-entry mistakes in the document.

「故障した機械を直すため、専門の業者を呼んだ。」
We called a specialist to fix the broken machine.

In more formal writing, you may also see these expressions:

• 修正する (shūsei suru / to revise or modify)
• 訂正する (teisei suru / to correct an error)
• 修理する (shūri suru / to repair)

For example:

「文章を修正する。」
To revise a text.

「間違いを訂正する。」
To correct a mistake.

「機械を修理する。」
To repair a machine.

「直す」 is a broad, everyday verb that can cover all of these situations. The more formal verbs describe the type of correction more precisely.

Medical and Health-Related Writing

In articles about health, medicine, or treatment, 「治す」 is the natural choice.

「この病気を治すためには、早期の治療が必要です。」
Early treatment is necessary to cure this illness.

「虫歯を治すには、歯科医院での治療が必要です。」
Dental treatment is necessary to treat a cavity.

「けがを完全に治してから、練習を再開してください。」
Please resume training only after your injury has completely healed.

In news reports and medical explanations, you may also see 「治療する」 (chiryō suru / to treat medically) or 「回復させる」 (kaifuku saseru / to restore or help recover).

However, combinations such as 「病気を治す」 and 「けがを治す」 remain natural in both conversation and general writing.

When you encounter 「治す」 in a JLPT passage, look for nearby words such as:

• 病気 (byōki / illness)
• けが (kega / injury)
• 薬 (kusuri / medicine)
• 医師 (ishi / doctor)
• 治療 (chiryō / treatment)
• 健康 (kenkō / health)

These words can help you identify the intended meaning.

Situation 3: When Feelings and Judgments Are Involved

The difference becomes more subtle when discussing habits, behavior, or personality.

Both kanji may sometimes appear with 「癖」 (kuse / habit), but they can create slightly different impressions.

Correcting a Behavior

「遅刻する癖を直したほうがいいよ。」
You should correct your habit of being late.

Here, the speaker sees being late as a behavior that should be changed.

「話すときに下を向く癖を直したい。」
I want to correct my habit of looking down when I speak.

「直す」 presents the habit as an action or behavior that can be adjusted.

For ordinary habits and behavior, 「癖を直す」 is the more common and neutral expression.

Overcoming a Deeply Rooted Problem

「子どものころからの悪い癖を、本気で治したい。」
I seriously want to overcome the bad habit I have had since childhood.

「治す」 can also be used with a habit. In this case, the habit may be viewed as a deeply rooted problem that is difficult to overcome.

The speaker may feel that changing it requires time and effort, almost like treating an illness.

Compare the impressions:

「悪い癖を直す。」
To correct a bad habit.

「悪い癖を治す。」
To overcome or cure a deeply rooted bad habit.

This difference is not absolute, but 「治す」 can make the problem sound more serious.

Be Careful When Talking About Personality

Consider this sentence:

「あなたはその性格を治したほうがいい。」
You should cure that personality of yours.

This may sound harsh because it treats the other person’s personality as if it were an illness.

A softer expression would be:

「その考え方は、少し変えたほうがいいかもしれないね。」
You might want to change that way of thinking a little.

「その話し方を少し直したほうがいいと思うよ。」
I think you should adjust the way you speak a little.

Choosing the correct verb is not only about grammar. It can also affect how polite, critical, or emotional the sentence sounds.

A Simple Summary of the Difference

The easiest way to distinguish 「直す」 and 「治す」 is to look at what is being improved.

Use 「直す」 when:

• Fixing a broken object
• Correcting a mistake or answer
• Revising writing or information
• Adjusting posture or pronunciation
• Improving behavior or an ordinary habit

Use 「治す」 when:

• Curing an illness
• Healing an injury or wound
• Treating a cavity
• Overcoming a habit viewed as a serious or deeply rooted problem

A useful memory rule is:

「壊れた物や間違いなら、直す。」
Use 「直す」 for broken objects and mistakes.

「病気やけがなら、治す。」
Use 「治す」 for illnesses and injuries.

Check Your Understanding with Example Sentences

1. 「壊れたパソコンを直してもらいました。」

I had my broken computer fixed.

This is natural when a machine or electronic device is repaired.

2. 「作文の間違いを赤いペンで直しました。」

I corrected the mistakes in my composition with a red pen.

Use 「直す」 for mistakes in schoolwork, tests, emails, and documents.

3. 「先生に言われて、座る姿勢を直しました。」

My teacher told me to correct my sitting posture.

Although posture relates to the body, it is not an illness or injury. Therefore, 「直す」 is used.

4. 「発音を直すために、何度も練習しました。」

I practiced repeatedly to correct my pronunciation.

This is a common expression in language learning. Pronunciation is adjusted or corrected, so 「直す」 is appropriate.

5. 「薬を飲んで、早く風邪を治したいです。」

I want to take medicine and get rid of my cold quickly.

A cold is an illness, so 「治す」 is used.

6. 「試合までに足のけがを治さなければなりません。」

I have to heal my leg injury before the match.

This is natural in sports and health-related situations.

7. 「来週、歯医者で虫歯を治してもらいます。」

I will have my cavity treated by a dentist next week.

A cavity is a health problem, so 「治す」 is the natural choice.

8. 「夜更かしする癖を直したいと思っています。」

I want to break my habit of staying up late.

The speaker sees this as a lifestyle habit or behavior to change, so 「直す」 is natural.

9. 「長年の悪い癖を治すのは簡単ではありません。」

It is not easy to overcome a bad habit developed over many years.

「治す」 makes the habit sound deeply rooted and difficult to change.

10. 「体調を治すために休みます。」

I will rest in order to cure my physical condition.

The meaning is understandable, but this sentence is not very natural in Japanese.

For 「体調」 (taichō / physical condition), use expressions such as:

「体調を整えるために、今日は早く寝ます。」
I will go to bed early today to improve my physical condition.

「体調が回復するまで、しばらく休みます。」
I will rest for a while until my condition improves.

Do not assume that every health-related noun can be used with 「治す」.

How to Read These Words on the JLPT

When 「直す」 or 「治す」 appears in a JLPT reading passage, do not look only at the verb.

First, identify the noun connected to it.

Words commonly used with 「直す」 include:

• 機械 (machine)
• 故障 (breakdown)
• 文章 (text)
• 間違い (mistake)
• 答え (answer)
• 姿勢 (posture)
• 習慣 (habit)
• 態度 (attitude)

Words commonly used with 「治す」 include:

• 病気 (illness)
• 風邪 (cold)
• けが (injury)
• 傷 (wound)
• 虫歯 (cavity)
• 薬 (medicine)
• 医師 (doctor)
• 治療 (treatment)

However, do not rely on the noun alone.

For example, posture relates to the body, but Japanese speakers say:

「姿勢を直す。」
To correct one’s posture.

They do not normally say:

「姿勢を治す。」

A habit can also be written with either kanji depending on the writer’s attitude.

If the habit is presented as an ordinary behavior that should be changed, 「直す」 is likely.

If it is presented as a serious, deeply rooted problem, 「治す」 may be used.

The key is to examine the surrounding context and understand how the writer views the problem.

Conclusion

「直す」 and 「治す」 share the pronunciation 「なおす」 and both express the idea of improving a bad condition. Their natural usage depends on the object.

• Use 「直す」 for objects, mistakes, writing, posture, pronunciation, and behavior.
• Use 「治す」 for illnesses, injuries, wounds, and cavities.
• In spoken Japanese, the two words sound identical.
• In written Japanese, choose the kanji based on what is being improved.
• With habits, 「直す」 is generally neutral, while 「治す」 can suggest a deeper or more serious problem.

Learning the dictionary meaning is only the first step. To use Japanese naturally, you also need to notice the situation, the object of the verb, and the speaker’s attitude.

At RJT — Rapid Japanese Training, you can practice JLPT vocabulary and grammar through short, focused questions. By repeatedly choosing the right word in context, you can build the ability to recognize subtle differences like 「直す」 and 「治す」 in real conversations and reading passages.