When it comes to pumping iron in the land of the rising sun and the city that never sleeps (Tokyo), there are a few options. Gold's Gym ^ is here, Konami gym ^ and Tipness gym ^, just to mention some. Personally I have only tried out one gym over here, so far, the Rondo Fitness Club. To find out where these gyms are located, Google Maps can help, if you search in Japanese. Example, do search for "ジム 東京 ^" (gym tokyo), or "ウェイトトレーニング ジム 東京 ^" (weight training gym tokyo). Or do the latter search on google.com ^. Tried searching for "powerlifting" as well, which only gave one result, Japan Powerlifting Association.
Alternatively, you can do like me, some light training at your own place. If you have a couple of rubber bands and a wheel for abs you can at least maintain some strength during your Tokyo stay.