PURPOSE: To obtain a stable optical signal transmitter where the extinction ratio is kept to an optimum value by keeping stably an output level of a transmission signal even when ambient temperature or the like of an external modulator is changed.

CONSTITUTION: An output light 2 of a semiconductor laser 1 is made incident to a waveguide type optical modulator 3, and waveguide outputs 5, 6 are divided into the 1st reference light 9, a signal light 10 and the 2nd and 3rd reference lights 11, 12 by beam splitters 7, 8. The lights 9, 11 are subject to photoelectric- converted and the sum is compared with the 1st reference signal 18 by an output fluctuation detecting circuit 20 and an error signal 19 is obtained. The signal 19 controls a temperature control elements 22 via a temperature control circuit 20 so as to adjust the output of the laser 1 thereby stabilizing the output of the modulator 3. The light 12 is led to a photodetector 24, compared with the 2nd reference signal 27 to keep the extinction ratio to an optimum value by using a comparator 26, an extinction ratio error signal 28 sets the bias level of the modulator 3 to an optimum value via a modulator bias circuit 29 so as to keep the extinction ratio to the optimum value at all times.