Medical fees
If you require medical treatment but do not have medical insurance, you must pay the full amount of the bill that can sometimes be very expensive.
There is medical-fee assistance for those suffering with incurable diseases and for the elderly, infants and single-parent families.

Japanese public medical insurance
In Japan, every citizen has an obligation to be covered by a public medical insurance in accordance with the insurance system.
There are 2 types of public medical insurance; the health insurance which one joins at one’s place of employment and the national health insurance which one joins through the city, ward, town or village office where one lives.
Being covered by medical insurance of a private insurance company is not a reason not to join a public medical insurance system.

If you are covered by public medical insurance, you must pay insurance premiums. However, if you become ill or have an injury, as a general rule, you need only pay 30% of the medical fees. You can also receive a refund after paying fees for childbirth or expensive medical treatment.

Note 1 For details, ask a person who understands Japanese to enquire for you at your nearest fire department, hospital or your city, ward, town or village office.
Note 2 The information given is that confirmed as of July 2002. Note that a new system or system revisions may have been introduced after this date.

This information is from the CLAIR website. The Council of Local Authorities for International Relations (CLAIR) was established in July 1988 in response to rising concerns about local level internationalization in Japan.