In a waveguide 10 including a substrate 1, a lower clad 2, an upper clad 3, and a core 4, the outline of a cross section perpendicular to the light propagating direction of the core 4 surrounded by the lower clad 2 has a shape curved with respect to the center of the cross section. The separation in the waveguide is thereby prevented, and the reliability of the optical property is enhanced.