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 obtained by injecting H ions on the surface of a lithium niobate crystal base or lithium tantalate crystal base, and irradiating it with electron 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 gold thin film 9 is vapor-deposited on the ion-exchanged layer 8 in order to prevent rise of electrostatic charge caused by electron beam scanning, and its surface is scanned with electron beams 10, and then, the film 9 is removed with an etching soln. The parts of the surface of the light guide thus obtained, scanned with the electron beam are formed into the grating changed in refractive index.