There are two types of motor vehicle insurance - Compulsory Automobile Liability Insurance (jibaiseki), and voluntary insurance.
The law requires every owner of a motor vehicle (two- or four-wheeled) to purchase the former insurance. The owner of a vehicle must pay the insurance premium at the time of the vehicle inspection, or shaken, together with the vehicle inspection charge and the vehicle tax. The insurance remains valid until the next inspection date.
Voluntary insurance is, of course, not mandatory. However, it is recommended that you purchase voluntary insurance, as damage compensation in Japan tends to be quite costly.
For details on both kinds of insurance, contact an insurance company or the association below.

The General Insurance Association of Japan
Sakaemachi Bldg. 6F, Nishiki 3-23-31, Naka-ku, Nagoya 460-0003
Phone: 052-971-1201