According to an embodiment, a semiconductor device includes an analog/digital conversion unit, a pulse width modulation unit outputting a transmission signal, the transmission signal being a pulse pattern corresponding to a digital signal output from the analog/digital conversion unit, a reference signal generation unit generating a reference signal, the reference signal being a fixed pulse pattern. The device includes a first control unit selecting one of the transmission signal and the reference signal, a light emitting element drive unit outputting a drive current based on the transmission signal or the reference signal, a light emitting element driven by the light emitting element drive unit. The device includes an optical receiving unit converting the optical signal into a voltage signal, and a demodulation unit demodulating the voltage signal into a digital signal based on the transmission signal or the reference signal.