PURPOSE: To enable to perform a reliable detection of a position of an object when two objects conduct a relative movement, by detecting a position in the proximity of a peak of an output of a light collecting element.

CONSTITUTION: When an output waveform (a) of a light collecting element 11 and a terinal voltage waveform b of a parallel CR circuit 13 and obtained, the rise of the waveform (b) is low by a full forward voltage, and a fall is delayed by the circuit 3, being higher than that of the waveform (a). As a result, a signal waveform (c) is obtained at an output terminal of a comparator 14. The B-point position of the signal (c) has no connection with a change in the signals (a) and (b), and a B-point is adjusted to approximate the peak of the signal (a) by means of a variable resistor 6 to set it as a reference position.