In between Marunouchi-building and Shin-Marunouchi-building which
are the centre of the Tokyo-Marunouchi area there is a gallery for free. It's located exactly between the both, and under the Gyoko street which the empire place to the special gate for Emperor of Tokyo station. Actually it's an isle connecting between Marunouchi metro station and Nijuubashi metro station. How practical it is. You may see some art-works when you just pass through the isle for the train.
The permanent exhibition is about photography of the past at Marunouchi called "Marunouchi in Pictures". You may enjoy the past and present of Marunouchi by the photos. That's the permanent one, but in many occasion there is exhibition collaborated with event around the area. And sometime's different like on 22nd
June, 2012. There will be a market called "Marunouchi Gyoko Marche". So you may buy veggies and anything good at the centre of a business district. It's quite interesting. So, just check it out when you pass through.
Gyoko Underground Gallery Official WEB-Site(Japanese) : http://www.gyokochika.com/