A light-emitting unit outputs an optical signal corresponding to an input electric signal. A light-receiving unit is electrically insulated from the light-emitting unit and outputs an electric signal according to the received optical signal as an output signal. In the light-receiving unit, a first light-receiving device outputs an optical current according to the optical signal. A second light-receiving device is provided not to receive the optical signal. A current duplication circuit duplicates a current flowing through the second light-receiving device. A current-voltage conversion circuit converts a current, which is generated by subtracting the current duplicated by the current duplication circuit from a current flowing through the first light-receiving device, into a voltage signal. A comparator output a result of a comparison between the voltage signal converted by the current-voltage conversion circuit and a threshold voltage as the output signal.