PURPOSE: To prevent the occurrence of malfunction due to a weak noise by binarizing the output signal of a light receiving means, and providing a gate means for taking out the binarized signals only over a valid measurement zone.

CONSTITUTION: The output signals of a light receiving element 8 and output signals after the binarization thereof can be obtained through the repeated scanning of a laser beam 5. The waveform of the output signals of a light receiving element 21 are shaped at a waveform shaping process 22, thereby obtaining a reference signal position over an invalid measurement zone. A timer 23 generates a pulse signal having a time interval so set as to rise at the trailing edge of the reference signal and to fall at the end of the invalid measurement zone. Another timer 24 generates a pulse signal having a time interval so set as to rise at the trailing edge of the output signal of the timer 23 and to fall at the end of a valid measurement zone, or a gate signal. As a result of sending gate signal to a gate circuit 25, the output signals of a binarization circuit 10 only over the valid measurement zone are selectively taken out and sent to a data processing circuit. According to this construction, the effect of noises generated near emission and receiving windows can be eliminated.