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 μm). This grating has achieved a high diffraction efficiency of 15.8%, which was about 10 times greater than before chemical etching.