Japan Health Insurance -before you see dentists-

Subscribers to the National Health Insurance System only need to pay 30% of the total medical fees if the insured receives treatment for an illness or injury.
However, the National Health Insurance does not cover all treatments and there are instances when the insured must pay all the fees.

Examples are when the insured is hospitalized in a private room in a hospital and must pay an extra charge for the bed, when the insured has received costly and special treatment that is not covered under National Health Insurance or when the person has received special dental treatment such as gold caps. Childbirth and abortions are also not covered by the National Health Insurance.

Japan Health Insurance Card (example)

You are fundamentally required to join the National Health Insurance System upon your arrival in Japan. If there is a delay in your application, you may be requested to make payments for past insurance fees due.

(from Studying Japan http://www.studyjapan.go.jp/en/faq/faq04e.html)