Sumo Museum & Kokugikan, Ryogoku, Tokyo

Sumo Information in general provided by Nihon Sumo Kyokai.
http://sumo.goo.ne.jp/eng/index.html

Even if you are not a big Sumo fan or if there's no tournament at Kokugikan, Sumo Museum is a good spot to stop after Edo-Tokyo Museum or before you head to chanko nabe lunch or dinner. It is free to get in and a good spot to learn history of Sumo. They also sell silly Sumo souvenirs such as magnets, towels and T-shirts.

About Sumo Museum

Location: first floor of The Ryogoku Kokugikan
Open: 10am - 4:30 pm
Closed: Saturdays, Sundays, National Holidays
Admission: free