PURPOSE: To enable to allow a light beam frequency to coincide with a reference frequency completely in a real time basis by always comparing the light beat frequency with the reference frequency formed of a crystal oscillator in a real time basis, and electrically processing the electric control amount without the force of a mechanical element.

CONSTITUTION: Two polarized light beam signals which cross perpendicularly of oscillated lights of a laser discharge tube 1 are converted to a voltage propertional to the frequency in a frequency/voltage converter 5 formed in a pulse by a waveform shaper, the voltage is compared with a voltage formed by a quasi- reference voltage frequency setter 7, the differential voltage is amplified by an amplifier 6, then applied through a power amplifier 20 to a motor 8 of a blower, and the temperature of a laser discharge tube is controlled so that the differential voltage becomes substantially constant. The output pulse of a waveform shaper 4 is inputted to one input terminal (a) of an RS flip-flop 11, and compared with a signal of an oscillator 9 of a crystal oscillator or the frequency of the reference frequency signal counted down by a counter 10 to be inputted to the other input terminal (b) in a real time in parallel with the above control.