One of the entrances to the huge Otemachi station

The entrance where you entered seems to be here .
[MAP] http://map.yahoo.co.jp/pl?nl=35.40.55.141&el=139.45.55.274&la=1&fi=1&sc=3

This station becomes the huge underground passage which was connected to a neighboring station .
You will be lost surely when you enter without attention . B-)