An integrated optical waveguide has first, second, and third optical waveguiding regions. The second optical waveguiding region has a refractive index different from that of the first optical waveguiding region. The second optical waveguiding region has an interface surface with the first optical waveguiding region that is inclined with respect to the waveguide direction of the first optical waveguiding region. The third optical waveguiding region has an interface surface with the second optical waveguiding region that is disposed such that a refraction direction through the interface surface is in line with the waveguide direction.