There is provided a monolithic photocoupler which is easy to integrate. An SOI structure is formed by providing a first insulation layer on a silicon substrate. The semiconductor single crystal region is further divided by trench insulation layers into separate regions. Light emitting elements are formed on one of the separated semiconductor single crystal region and light receiving elements are formed on the other semiconductor single crystal region. The light emitting elements are obtained by forming light emitting diodes made of GaAs or the like on the substrate using a heterogeneous growth process. An optical waveguide made of a material which is optically transparent and electrically insulative such as a TiO.sub.2 film on each pair of light emitting and light receiving elements. The insulation layers constituted by SiO.sub.2 layers have a refractive index smaller that of the active layer of the substrate. Thus, the light emitted by the light emitting elements is almost entirely reflected without leaking out, and the light which has entered the optical waveguides reaches the light receiving elements without leaking out except those components which have entered perpendicularly or at an angle close thereto.