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I observed this fly back in March on a sea shore. This small fly was relatively numerous. Reminded me some Platypezidae spp, but when looking at the images, these flies had quite differently structured heads from the Platypezidae spp. observed by me earlier.

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Try to see which species of Lipara are found in Japan and check those of. They are the gall inducing Chloropidae on Phragmites.

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I found these flies invariably on a plant very different from a Phragmites. maybe some Umbelliferae. Also there were no reeds anywhere near the place, since it was a marine shore habitat, although there were some sizable dune grass species around.

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I will see if there are similar genera around. The family identification is certain, though.

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Calamoncosis duinensis (Strobl, 1893), Chloropidae. The coastal biotope, the peculiarly whitish wing membrane, the semicircular scutellum without tubercles, the ocellar triangle longer than 2/3 of the frons, and the mentioned small size of the fly speaks for this taxon though the body incl. the scutum should be black. May be, a recently emerged and not yet full coloured specimen or the flash light was too strong.

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In Japan, there is also Hecamede (family Ephydridae). I think it is something like that or close.

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My impression is that this is the adult color. There were many similarly looking flies, and all of them looked like this.

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I agree with Niels-Jan. This is Hecamede. - H.albicans and H.granifera both occur in Japan.