PURPOSE: To easily form a pattern large in equivalent refractive index change amt. and having uniform roughness on the whole face on the surface of a light guide by forming an ion exchange layer on the surface of a lithium noibate crystal base or lithium tantalate crystal base, and irradiating it with leaser beams.

CONSTITUTION: The x-plate or z-plate of LiNbO3 or LiTaO3 crystal 1 is heat- treated in heated benzoic acid to cause ion exchange between both; to diffuse Li ions 6 in the crystal 1 into the outside and H ions 7 into the inside of the crystal 1. The ion-exchanged crystal 1 is washed with ethanol by using ultrasonic waves. A metal, semiconductor, Ga, or org. thin film 9 for light absorption is vapor deposited on the ion-exchanged layer 8, and its surface is scanned with Ar laser beams 10 having a small spot to form a pattern. After the scanning, the film 9 is removed with an etching soln. or a solvent. It is found by observation that the parts of the surface of the obtained light guide scanned with the laser beam spot are relieved and equivalent refractive index change is obtained, thus permitting optical defects to be scarcely caused, and a light guide having good light guiding characteristics to be obtained.