PURPOSE: To suppress noise and distortion at a low frequency range, by converting a part of semiconductor laser output light into an electric signal at a photo diode, and feeding it back negatively to a current driving the semiconductor laser at a frequency region lower than a carrier.

CONSTITUTION: A part of an output light of the semiconductor laser 13 is received at a photodiode 16 and converted into an electric signal, the difference between an output after passing through a low pass filter 17 and an output passing an original input signal through a low pass filter 18 is amplified at an amplifier 19 and fed back negatively to a driving current of the semiconductor laser 13. Then, the noise and distorted output of the semiconductor laser light at a low frequency region produced due to the reflected return light and the confliction of the oscillating mode of the semiconductor laser 13 are reduced.