Correct Louis It is a Sphaerocerid. Note the spur at the end of the hind tibia and the large,wide metatarsus.I will try for a genus next.It looks familiar.Robert
Sphaeroceridae was my guess too, but this family seems to be relatively scarce in Japan, so I had no experience. Some recent threads with apterous flies show very similar flies, the wings in this species also seem to be somewhat short.
Xespok Hence the big legs. They aren't great on the wing. Interesting about Japan Robert PS. There is a pic in the gallery and more info on http://en.wikipedia.org/wiki/Sphaeroceridae Robert