A double-structure silicon on insulator (SOI) substrate with a silicon layer, an insulation film (silicon oxide film), a silicon layer, and an insulation film in this order from the side of the surface. The upper-layer insulation film is formed so as to have a uniform distribution of depth while the lower-layer insulation film is formed so as to have a non-uniform distribution of depth so that a thick portion may be formed in the silicon layer along a predetermined path. The refractive index of Si is 3.5 and the refractive index of SiO2 is 1.5. The thick portion of the silicon layer provides a core and the insulation films corresponding to this thick portion provide clads, thereby forming an optical waveguide along the predetermined path. The silicon layer at the side of the surface has a uniform thickness, thereby enabling characteristics of MOS devices fabricated on various portions of the silicon layer to be met with each other easily and facilitating a design of the electrical device as a whole.