Your WWOOF Japan membership does NOT include insurance of any kind.   We recommend that you arrange insurance for yourself to cover yourself for injury.  Some people say that it is unlikely that one will be injured.  To our knowledge nobody has been injured while WWOOFing in Japan in the past.  But we recommend you take insurance against injury.  Depending on what country you are in, there are different products available, often from travel agents.  You should make clear on your injury insurance application the details about WWOOF, that it is an exchange program giving you meals and accommodation in return for your helping the host with the work they do, and that many hosts are farms.