Solid-state photocycloaddition reactions of 6, 6'-dimethyl-4, 4'-polymethylenedioxy-di-2-pyrones (la-e) with benzophenone (2a) gave the corresponding oxetane derivatives (3a-e; 1:2 adducts) with high site- and regioselectivities across the C5-C6 and C5'-C6' double bonds in 1 via the triplet excited state of benzophenone. Since the yields of oxetanes between 1b,d (methylene chain length n=3,5) having odd methylene chains and 2a in the solid state were higher than those of la,c,e (n=2,4,6) having even methylene chains, it was suggested that the former mixed crystal formation is easier than the latter case. The oxetane formation proceeded more effectively in the solid state than in solution.