If you go up to any volunteer situation that includes any risk at all, it appears that you should have insurance.Some NPO offer insurance to volunteers, but others do not, and then again, others are a bit unsure. It seems better to have some on your own. It is easy and cheap to get.

Even if you have other health insurance (some of which might not cover you if you knowingly go into a risk situation),this insurance not only covers you if you are injured or hurt while volunteering,it covers your liability if you cause someone else to get insured, or if you damage somebody else's property.

Death - 14,000,000 yen
Injury - 14,000,000 yen
Hospitalization - 7,000 yen
Surgery - one procedure is covered, payment depends on type of surgery Doctor visits - 4,100 yen Liability (covers harm to other people or property - 500,000,000 yen

Looks like a one-year enrollment in volunteer insurance with natural disaster coverage is 490 yen. You can get slightly more coverage for 720 yen. You can enroll at your nearest branch of the Japan National Council of Social Welfare.

http://www.shakyo.or.jp/index.htm

Here is the address and info for the Tokyo office, but I think that you can apply at any local ward office.

Fukushi Hoken Saabisu (Welfare Insurance Service)
〒100-0013 Tokyo-to Chiyoda-ku Kasumigaseki 3-3-2 Shin-Kasumigaseki Biru Tel. 03-3581-4667 Fax. 03-3581-4763

Information thanks to Camellia Neih