PURPOSE: To improve the precision of an apparatus for measuring distance and oscillation by the use of an optical beat, by comparing the amplitude of a frequency of a laser with that of a frequency of electric oscillation so as to judge positive or negative.

CONSTITUTION: The beat of perpendicularly-intersecting beams of a laser 1 having a frequency fo is detected by a detector 10, while the beat signal of perpendicularly-intersecting polarized lights to which a Doppler shift fb of a moving mirror 5 is applied and which has a frequency fs is detected by a detector 11. These two electric signals are converted into pulses corresponding to the respective frequencies by wave shaping circuits 12 and 13 and inputted to the set and reset terminals of a flip-flop 14 respectively. When there is even a slight difference between the frequencies of the two inputs, a time interval between the set and reset is varied, and when the width of a pulse delivered from an output Q reaches a prescribed value, it turns to be zero immediately. By taking the outputs of integrating circuits 15 and 16 through differential circuits 17 and 18, only a portion varying sharply can be taken out.