An active layer has a quantum well structure formed of InGaAsP, and includes a saturable absorption region and optical amplification regions. To the saturable absorption region, a voltage is applied through a p-electrode, independent from the optical amplification region. To the optical amplification regions, currents are injected through p-electrodes, respectively. An input light Pin entering through a plane of incidence is generated by adding optical noise of white noise, to a light signal assuming binary optical intensity of “1” or “0”. The saturable absorption region and optical amplification regions are formed satisfying conditions that a waveform converting element provides a semiconductor laser of bistable state.