To relatively easily form core patterns of high accuracy.

This method consists in forming an optical element mounting region 14 on a flat p-type Si wafer 10 as a substrate and implanting an impurity to the front surface part of the p-type Si wafer in this optical element mounting region 14 to form an n-type Si layer 18. The front surface part of the Si wafer is denatured to a porous Si layer 22 by executing anodic oxidation. Since the n-type Si layer is formed in the optical element mounting region, the region is not made porous. In succession, a clad layer 24a, a core pattern 26a and an (upper) clad layer 30a are laminated on the porous Si layer and a flat optical waveguide is formed in the waveguide forming region 16 of the substrate. The porous Si pattern 22c formed by patterning the porous Si layer in such a manner constitutes the lower clad layer together with the clad layer 14a. An insulating layer 34 is deposited over the entire surface of the substrate. The optical element mounting region of the substrate is provided with a die bonding pad 36 and a LD(laser diode) 38 is mounted thereon.