An optical semiconductor device includes a semiconductor support substrate of a conductivity type having a first resistivity, a semiconductor layer of the conductivity type formed on the semiconductor support substrate and having a second resistivity higher than the first resistivity, a first power supply terminal having a first metal in Schottky barrier contact with the semiconductor layer along with the semiconductor support substrate, so that a Schottky diode element is constructed by the first power supply terminal and the semiconductor layer along with the semiconductor support substrate, a second power supply terminal having a second metal in ohmic contact with the semiconductor support substrate, and a semiconductor light-emitting element connected between the first and second power supply terminals, the semiconductor light-emitting element being antiparallel with the Schottky diode with respect to the first and second power supply terminals.