PURPOSE: To stably keep the extinction ratio and the light intensity modulation amplitude by feeding back negatively an output of an optical frequency discriminator converting a frequency modulation component of a signal light into an intensity modulation component to a light source section or an optical frequency discriminator.

CONSTITUTION: An output light 20 of a semiconductor laser 1 of wavelength 1.5μm subject to direct pulse current modulation of 1Gb/s is inputted to a Mach- Zehnder interferometer 6 comprising a half mirror 5 and a mirror 4 and acting like a frequency discriminator. Part of the output light 9 converted into the intensity modulation component by the itnerferometer 6 is inputted to a photodetector 7. Then the voltage output of the photodetector 7 aud the voltage of a DC power supply 11 are made coincident when the extinction ratio of the output light 9 is 1:100 to make the output of a differential amplifier 8 zero. The differential amplifier 8 regulates the titled transmitter that the temperature of the laser 1 is decreased when the output of the photodetector 7 is increased thereby increasing the oscillating frequency.