PURPOSE: To manufacture a pattern for thermal diffusion without using a resist and a mask by partially heating a metal thin film formed on a base consisting of a ferroelectric body to form an oxidized area, and using the oxidized area as the pattern for thermal diffusion.

CONSTITUTION: A metal thin film consisting of Ti is uniformly formed on a base 12 consisting of LiNbO3, for example, by evaporation. A laser beam is emitted to a determined part of the metal thin film 14 by use of a YAG laser device to heat it, and an oxidized area 16 is formed. The base having the metal thin film 14 and the oxidized area 16 formed thereon is successively dipped in an etching solution for Ti. The non-oxidized part of the metal thin film 14 is removed by etching, but the oxidized area 16 is hardly etched because of its low etching rate, and left as it is to form a pattern. The base having the oxidized area 16 left thereon is heated, for example, at a temperature of 1000°C or more, whereby the oxidized area 16 is thermally diffused in the base 12 to form a high refraction index part 18 on the base 12.