Do you need to be in good health to naturalize?

Some countries require you to submit your medical history and/or be in good health for naturalization. While Japan does require people that are applying for 永住権 {eijūken} (permanent residency) to be in good health, there is no such requirement for naturalization. Also, unlike many other countries, Japanese permanent residency status is not a prerequisite for obtaining Japanese citizenship.

In case you were wondering, the good health requirement for Japanese 永住権 {eijūken} (permanent residency) is defined with the following guideline:

公衆衛生上 {kōshū eisei jō}有害となる {yūgai to naru}おそれのある疾病 {osore no aru shippei}（梅毒その他 {baidoku sonota}伝染性疾患 {densensei shikkan}及び {oyobi}麻薬、覚醒剤等の中 {mayaku, kakuseizai nado no naka}毒性疾患 {dokusei shikkan}）に罹患していないこと。 {ni rikan shite inai koto.}健康診断書等を {kenkō shindan sho nado o}提出します。 {teishutsu shimasu.}

Which roughly translated / paraphrased, is:

Those who are not suffering from diseases that are harmful to the public health (including syphilis and other infectious diseases, as well as addictions to drugs such as stimulants or narcotics). You may have to present a health examination certificate.

There is no health requirement for 特別永住権 {tokubetsu eijūken} (Special Permanent Residency), which is not a status that is available to most immigrants anyway.

Prior to naturalizing, I had permanent residency status in Japan, but I was not required to submit any documentation related to my health when I applied, so I guess the requirement is at the discretion of the case worker handling the applicant.

It is conceivable that if you have an obvious problem with your health that would prevent you from working in most areas of employment, they may ask additional questions about another requirement for naturalization, which is:

自己又は {jibun mata wa}生計を {seikei o}一にする {itsuni suru}配偶者 {haigūsha}その他の親族の資産 {sono ta no shinzoku no shisan}又は技能によって {mata wa ginō ni yotte}生計を営むことができること {seikei o itonamu koto ga dekiru koto}

Or paraphrased:

be able to, with your skills and the combined financial resources of you and your spouse & family, support yourself

By the way, while it's true that Japanese health care is pretty good by most world standards according to international metrics and that Japanese have some of the highest longevity rates in the world, it is known that foreigners who immigrate as an adult and live in Japan for a long time do not seem to inherit this longevity trait.

It is unknown whether naturalized citizens live as long as non-naturalized Japanese, probably because there is not yet enough data.