PURPOSE: To produce a small-sized diffraction grating having high diffraction efficiency by exposing crystal faces of single crystal layers other than the growing faces, selectively etching one of two kinds of single crystal layers appearing on the exposed crystal faces, and using the remaining crystal faces as diffraction faces.

CONSTITUTION: A gallium arsenide crystal layer 12 having 1,000 thickness is grown on the (100) crystal face 11 of gallium arsenide GaAs, and an aluminum gallium arsenide (Al0.2Ga0.8As) layer 13 having 200 thickness is further grown. After the growths are repeatedly carried out about 1,000 times, a gallium arsenide layer 14 having 300μm thickness is grown. The resulting wafer is coated with a photoresist pattern having slits parallel to the [01'1] direction, the (111) faces 15 are exposed with an anisotropic etching soln., and the aluminum gallium arsenide layers 13 are etched by about 200 with other etching soln. The exposed (111) faces are used as diffraction faces and 2,024 grating spacing, that is, 4,898 bars per 1mm are obtd. The wafer is then cleaved to 2mm square to obtain a minute diffraction grating.