Nonlinear optical copolymer with 4-dimethylamino-4’-nitrostilbene as a side chain group was investigated for the photostability by irradiation of infrared light. The light at 1.313 mu m, which is very low absorptive for the material, was coupled into the polymeric channel waveguide formed by photobleaching technique. Transmission loss of the infrared light through the channel waveguide has been negeligibly changed even after for a long irradiation. Also, the refractive index change in slab waveguide is undetectable. The nonlinear optical polymer used in this study turns out to be quite reliable in terms of photostability.