PURPOSE: To stabilize the frequency of laser light to the Lamb dip of the light absorption line of atoms or molecules by modulating the laser light by using the Doppler effect.

CONSTITUTION: The lamb dip of laser light from a semiconductor laser 1 can be observed from the intensity of the reflected light of the laser light by perpendicularly reflecting the laser light by means of a reflecting mirror 7 after the laser light is passed through an absorption cell 6 in which atoms or molecules are enclosed and again passing the reflected light through the cell 6. Since the frequency of the reflected laser light is modulated by a Doppler effect when the reflecting mirror 7 is vibrated, the differentiated waveform of the Lamb dip can be obtained when phase detection is performed on the intensity of the reflected laser light by using the frequency of the vibration of the mirror 7. The frequency of the laser light is stabilized to the peak frequency of the Lamb dip by feeding back the phase-detected output to a semiconductor laser controlling current source 12 as a frequency discriminating signal.