See example #1 from the [Digital Museum of Plugs and Sockets][1], and more pictures on other pages.  One style of outlet in Japan has a separate screw connection for the grounding wire, usually under a plastic flap.

For use elsewhere, this should be rewired to a standard three prong plug available at any hardware store (NEMA 5-15P for North America).  After all your hiney is right above water, ground is important (and I say that as a fan of keeping ungrounded K&T where appropriate http://diy.stackexchange.com/a/20279/5960 ).

  [1]: http://www.fam-oud.nl/~plugsocket/Japanese_3hd.html