PURPOSE: To reduce a residual voltage in a short time by supplying a bias current close to an oscillation threshold level to a semiconductor laser at reception so as to select the non-illuminance state from the illuminance sate with a forward bias thereby reducing an electron discharge time in the bias state.

CONSTITUTION: A transistor(TR) 61 of a bias circuit 62 in the reception state is turned on and shifted between two current values in the transmission state and the reception state and a current flowing even in the non-illuminance state of a semiconductor laser 42 is set. The current in the non-illuminance state is set to a threshold current of laser oscillation of the laser 42, which is used as a light receiving element as it is. The laser 42 has an amplification function to an external incident light when a bias current around the oscillation threshold level is supplied to the laser 42, and the density of electrons of semiconductor laser junction is decreased by induction radiation when any signal light is in existence. The circuit does not use a load resistor and an amplifier 64 amplifies directly a change in a junction voltage. As a result, the residual voltage of the semiconductor laser junction is momentarily decreased at transmission reception changeover.