PURPOSE: To realize the data communication and at the same time to form a system which has a reduced level of disturbance, by modulating the quantity of higher harmonic component of the optical output of a semiconductor laser by modulation of temperatures.

CONSTITUTION: A driving circuit 2 decides the bias of a semiconductor laser 1 by the output of a laser output monitor device 4 which is supplied via a storage circuit 3 and at the same time modulates the laser 1 with the periodic signal supplied from a periodic signal producing circuit 5. The device 4 detects the amplitude variations of the light given from the laser 1 which can not compensated completely by a cooling device 6 by making incident some of the optical signals given from the laser 1. The detecting signal of the device 4 is fed back to the circuit 2. The circuit 3 stores the detecting signal given from the device 4 by the communication starting signal given from a data communication signal producing device 7 and then releases the storage with a communication end signal. Then the detecting signal given from the device 4 is supplied directly to the circuit 2.