PURPOSE: To eliminate a decoupling phenomenon and to obtain high optical coupling efficiency by changing the boundary part of a coupling region and a non-coupling region to a curvilinear shape in a transverse direction orthogonal with a propagation direction of guided light, thereby forming the above coupler.

CONSTITUTION: An incident beam 5 introduced into a waveguide layer is made into an elliptic shape when this incident beam 5 is made incident through an aperture 8 into the optical coupler. The optical coupler is divided into the coupling region A where the guided light 7 is obtd. by coupling the incident beam 5 to the waveguide layer and the non-coupling region B where the incident light beam 5 exclusive of the coupling region A is not coupled to the waveguide layer. The Boundary part C of the coupling region A and the non-coupling region B is formed by changing this part to the curvilinear shape in the transverse direction orthogonal with the propagation direction Z of the guided light 7 in such a manner that the optical coupling efficiency is maximized according to the light intensity distribution 6 of the incident beam 5 and the eclipse quantity ΔK of this beam. As a result, the decoupling phenomenon near the boundary part C is eliminated.