PURPOSE: To surely measure a near distance and a far distance over a wide range while securing the measurement accuracy within a practical range by changing the output voltage of a drive signal generating circuit so that the received wave-form from a receiving element becomes a wave-form of a proper size.

CONSTITUTION: A time measuring circuit 62 starts the timing action based on the LD drive signal (a) from a drive signal generating circuit 11. The reception signal (b) is fed to a peak detecting circuit 61 from an amplifying circuit 17 whenever the signal (a) is outputted from the circuit 11, the peak of the received wave-form is detected, the peak signal (c) is fed to the time measuring circuit 62, and the counting action is stopped. The counted result is fed to a correcting circuit 63, and the corrected result is fed to a calculating circuit 41'. When an obstacle exists at a very near distance and a saturation phenomenon occurs on the received wave-form inputted to the circuit 61 and no peak point of the wave-form is detected, the circuit 61 feeds the control signal to an LD switching circuit 12' to lower the output voltage, and the luminescence output power is reduced.