A light modulation device comprises a substrate (1), a substrate layer (2), an optical waveguide layer (3) and buffer layers (4), in that order, formed of either all n-type or all p-type compound semiconductor crystal. In orderto capture the light in the optical waveguide layer, the composition ratio of the compound semiconductor is so determined that the refractive index is at least approximately 0.1% higher in the optical waveguide layers than in the substrate layer and in the buffer layers. Furthermore, the carrier density is low in the optical waveguide layer and in the buffer layers, so that the applied voltage is effectively applied mainly to the optical waveguide layer. Due to the construction of the light modulation device, strict control of the etching process is not required, the device has a low absorption loss of light, and it can be made as a monolithic optical integrated circuit.