To provide a optical integrated circuit with a small optical coupling loss.

This method is used for manufacturing an optical integrated circuit with at least two waveguides 7 and 3 of different constitution, and the ends thereof connected to each other. In this case, the waveguides 7 and 3 are separately defined and formed using inherent masks 10 and 12, so as to form the coupling ends for expanding the size of the lateral mode of optical waves emitted therefrom.