In Japan, national health insurance is compulsory, and all Japanese nationals, including foreigners who have lived for more than one year, must also register as foreigners. Holders of visa 1 year or more must join the national health insurance. In addition, although the initial residence period is less than one year, those who have a residence permit for more than one year must join the national health insurance. Then, it means that as an international student, you must apply for national health insurance.
Applying for national health insurance has many advantages. Not only do you enjoy the same medical treatment as Japanese nationals, but it is much cheaper to see a doctor, and a person has basic protection abroad. When you go to see a doctor, you must remember to bring your insurance certificate!
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.