In As‐Se‐ (S) ‐Ge amorphous films, a remarkable difference of chemical etching rate between the heat‐treated state and the light‐irradiated state has been observed. Utilizing this characteristic, a relief‐type diffraction grating of the amorphous film was obtained (the pitch between grating lines, 0.86 &mgr;m). This grating has achieved a high diffraction efficiency of 15.8%, which was about 10 times greater than before chemical etching.