PURPOSE: To prevent a detection mistake by noises, etc. and measure very accurately a distance by emitting pulses on a body to detect those reflected pulses and to store to a memory in order, and computing the distance to the objective body.

CONSTITUTION: An A/D converter 4 converts the output signal of a photodetector 3 at leading edges of clock pulses. A processing circuit 7 reads in order stored values of a memory 5, judges the maximum value among a series of stored values the peak of the pulse light reflected from a body 10, finds the time T from the emission of pulse laser 1 to the peak of the reflected light by that generated order, and converts it to a distance, when the operation from the A/D conversion to the storage into the memory 5 for the pulse emission of one time is finished. A detection mistake is prevented and the very accurate distance measurement can be made because the signal of the reflected pulse light as it stands is not compared with a prescribed threshold in this case.