In reply to Ayrton:
To meet other climbers: post on Tokyogaijins or head over to T-Wall Edogawabashi on a Thursday - usually a contingent there.
For climbing outside.
Ogawayama: 3.5hrs by car. Sport, trad and bouldering. Granite. ***
Jogasaki Kaigen: 2.5 hrs by train. Sport and a little trad. Sea cliffs. ***
Yugawara: 1.5hrs by train. Sport and a little trad. **
Mitake: 1hr or so by train. Bouldering by the river. *
Joyama: 2hr by train + little taxi ride. Multi and single pitch sport. **