The effect of photoirradiation on the refractive index of a diarylethene polymer was investigated. Diarylethene polymer was prepared by a radical polymerization using 1-[6＇-(methacryoyloxyethyloxycarbonyl) -2＇-methylbenzo[b]thiophen-3＇-yl]-2-(2 "-methylbenzo[b]thiophen-3 "-yl) hexafluorocyclopentene (BTF6MA), styrene, and butyl methacrylate. A colorless film prepared from the diarylethene polymer turned a deep red hue upon exposure to UV light. The red color was immediately bleached by visible light, and such coloration and bleaching of the polymer film could be switched reversibly by using two light source of UV and visible light. The refractive index (n(TE)) of the polymer film, with 5.3 mol % of BTF6MA, was determined as 1.5555 and 1.5459 by a prism coupler using TE polarized light of 633 and 830 nm, respectively. Upon irradiation with light of 365 nm, the refractive index was increased, resulting in a photoinduced refractive index change (Delta n(TE)) of 8 x 10(-4) as monitored by Light of 830 nm. Delta n(TE) was dependent on the wavelength of the monitoring light. The dispersion of light in the diarylethene polymer film could be simulated by the Sellmeier equation, n(lambda)(2) = n(infinity)(2) + D lambda(0)(2)/(lambda(2) - lambda(0)(2)), in which the refractive index at longest wavelength limit (n(infinity)) and the oscillator strength (D) for the colorless polymer film were determined as 1.5356 and 0.18, respectively. Similarly, n(infinity) and D values for the red polymer film, prepared by UV irradiation, were determined as 1.5368 and 0.042, respectively.