A novel optical device based on a ’’dynamical refractive‐index change’’ in an amorphous As2S3film is constructed. In this device coupling of a red light beam into the thin‐film waveguide with a prism coupler is controlled by illuminating blue light. A switching time of about 10 s and an on/off ratio as high as 6/1 have been achieved. Further, this device can be operated as a photo‐optical switch having a storage function by changing the intensity of the light beams.