PURPOSE: To detect the over-run of a back and forth moving body by flickering and energizing the originating element of an over-run detector and descriminating the over-run in accordance with the fluctuation in the pulsating reception signal.

CONSTITUTION: The LED of the photocoupler or the like 49 of an over-run detector 51 is flickered and energized with an oscillator. The pulsating signal received with the receiving element such as a phototransistor of the photocoupler 49 is fed through an HPF such as an operational amplifier OP 2, a smoothing circuit or the like to compare the signal with a set voltage, and an operational amplifier OP 3 generates a low level output. When the light from the LED of the photocoupler 49 is shielded in the position just before the over-run by the shielding end moving integrally with a back and forth moving body of a copying machine such as a mirror for exposure or the like, the output of the OP 2 goes zero, and the output of the OP 3 inverts to a high level, then a monostable multivibrator MM is triggered to set a flip-flop FF 1 or FF 2 and to detect securely the over-run.