The Nihon Ki-in (日本棋院) is the principal Japanese go association. It is based in Tokyo and was formed in July 1924. The main driver was an earthquake in 1923, which caused great financial hardship on the existing go groups. Among the innovations were the organisation of the Oteai promotion system, the introduction of time limits and the introduction of amateur dan diplomas. There is another professional organisation in Japan, based in Osaka: the Kansai Ki-in.
The Nihon Ki-in has a website, the English part of which you'll find here.
At this time I coulnd't read the name in kanji on the building, so I mistook this place for a restaurant, and I spend half an hour trying to figure out where I was wrong. Luckily, I met a nice and funny おじさん (ojisan) who claimed to be member of I do not remember which confraternity, and who can speak some English. He came with me inside the building and helped me to meet the person I was looking for. I love Japan! ^_^