PURPOSE: To stabilize an optical output at the existence of a pulse signal for monitoring use and to make the optical output variable at the time other than the existence of the pulse signal for monitoring use by a method wherein an identical modulation current flows at the existence of the pulse signal for monitoring use and the modulation current is changed at the time other than the existence of the pulse signal for monitoring use.

CONSTITUTION: A detection circuit 11 outputs a detection signal 12 when one part of a beam radiated from a laser diode 1 is made incident and a pulse signal 10 for monitoring use is in an 'ON' state; it holds the signal 12 when the pulse signal is in an 'OFF' state. A comparison circuit 13 compares the signal 12 with a reference voltage 14, and outputs a control signal 2. When the signal 10 is in the 'ON' state, an input pulse signal 7 is made to be identical to the signal and selects an identical modulation current 6. A control current circuit 3 is operated in such a way that an optical output at this time is used as a reference beam and that a control current 4 used to stabilize the optical output flows. When the signal 10 is in the 'OFF' state, the signal 12 is held and, accordingly, the control current 4 is also held. By this setup, when the input pulse signal is changed, the modulation current 6 is changed, a driving current of the laser diode 1 is changed and the optical output becomes variable.