They smoke plenty of weed in Japan.
I was only there for 2 nights and got smoked out by people I met each night. No clue about growing there.
It check out the laws if you can. I dont think Japan is as bad as Singapore or anything.
Also take in to account where you're living. Will probably be an apt since Japan has limited space. In some Asian countries, landlords are a lot more likely to stop by your place unannounced, escpecially if you are a foreigner.
I'd hold off on growing until you've been there awhile and get to know the local scene and culture.