PURPOSE: To improve the frequency reproducibility and stability of the title laser light source by sending a noise reducing signal to a laser controlling current source upon detecting frequency noise in the output light of a laser from the slope of a linear absorption signal.

CONSTITUTION: A control signal is generated by means of a laser center frequency stabilizing system 45 by using either one of lines near the central section of the slope of a linear absorption line of the saturated absorption spectra or crossover lines detected by a photodetector 40 and used for stabilizing the frequency of a laser 31 by feeding back the control signal to the laser 31 through a laser controlling current source 47. On the other hand, a noise signal discriminated by means of a linear absorption line obtained from another photodetector 44 is used as a control signal by means of a laser noise reducing system 46 and fed back to the laser through the controlling current source 47 for reducing noise. Therefore, a low-noise frequency-stabilized laser light source having a highly stable oscillation frequency and narrow line width is obtained.