PURPOSE: To make a waveform of a transmitted optical signal steep without decreasing the service life of a light emitting element by driving the light emitting element according to an input signal.

CONSTITUTION: The circuit is provided with the light emitting element 3 such as a light emitting diode or a semiconductor laser driven according to an input signal and an optical switch 4 controlled to be turned on/off according to the input signal and receiving light from the element 3. Since the light emitting element 3 is driven according to the input signal, the service life is not decreased. Further, the optical switch 4 cuts off at least the trailing edge of an output optical signal from the light emitting element 3 in the leading and trailing edges to transmit an optical signal with a steep waveform.