Photochromic ring-closure reaction of a network polymer film (Poly BPOH-C) obtained by oxidation polymerization of the closed-ring isomer (BPOH-C) of 1,2-bis[2-methyl-6-(o-hydroxyphenyl)-1-benzothiophen-3-yl]hexafluorocyclopentene (BPOH-O). In the film, diarylethene molecules are densely packed and they are fixed in photoreactive anti-parallel conformation by cross-linking of the polymer networks. The rate of the film was estimated to be less than 30ps. These values are similar to those of diarylethene monomers obtained in the solutions, in polymer dispersed system, and in crystalline states, indicating that the oxidation polymer film system keeps high response.