If you are a farmer in Japan, and would like to become a WWOOF Japan Host, at first you must send the stipulated papers along with a 140 yen stamp by post to the WWOOF Japan office. 

Please read the menu item "ホスト（受入先）になろう" in Japanese.  It explains what to do. 
A condition of becoming a WWOOF Host in Japan is that at least one person at the Host's place must speak and read Japanese fluently.  

Live organically!

Good WWOOFing!
The WWOOF Japan team.
Grass roots movement for organic food, sustainable lifestyles, and social justice.