A core, constituted by an amorphous undoped semiconductor (i type), which is formed on a lower clad layer, and a p-type layer and an n-type layer which are disposed on the lower clad layer with the core interposed therebetween and are formed in contact with the core are provided. The core is formed to be thicker than the p-type layer and the n-type layer. The p-type layer and the n-type layer are constituted by single crystal silicon.