...in west Shinjuku, also known as Tocho (都庁), 797 ft and 48 floors high. It's been there since 1990, day...
...and night and actually, part of it is the building on the right as well as one more building behind it, housing the Tokyo Metropolitan Assembly.
(For a bigger version of these pictures both in color and black and white, check my "Japan Arekore" set on Flickr).