In the upper face of a lower clad layer, a core trench is formed having a cross-sectional trapezoidal shape having a narrow bottom face and a wide upper face, a core is formed within the core trench, and an upper clad layer is formed at the upper face of the core for sealing. The bottom face of the core trench becomes a low refractive index layer having a refractive index smaller than the lower clad layer. The substantive difference of refractive index between the core and the lower clad layer is thus increased.