PURPOSE: To remove an error in distance measurement time due to the rounding of a signal and variation in its amplitude and to improve the measurement accuracy by providing a circuit which measures the pulse width of the output signal of a photodetector, a circuit which measures the pulse width of the output signal of a photodetection system, and an arithmetic circuit, and measuring the time interval between the centers of both the pulses.

CONSTITUTION: Light with ultra short pulse width is made incident on a half- mirror 2 from a Q switch pulsed laser oscillator 1, the majority of the laser light is transmitted, and the remainder is sent to a photodetector 3. The photodetector 3 converts the pulsed laser light into an electric pulse signal and a comparator circuit converts a level above a threshold value into '1' and others into '0'. The majority of the laser light is made incident on a transmission optical system 4, narrowed down in beam divergence angle, and sent out to a distance measurement target 5. The irradiating light 6 is scattered by the surface, but part of it is converged by a reception optical system 7 and a photodetection system 8 converts only the pulsed laser light into an electric signal; and the comparator circuit generates '1' and '0' based upon the threshold value and a counter 9 measures time intervals of '1'.