PURPOSE: To perform detecting operation with high sensitivity by setting the terminal voltage across a capacitor in a process wherein a capacitor is charged temporarily and then discharged, and setting a reference value in the discharging process of the capacitor.

CONSTITUTION: The reference value setting circuit 16 of a back member detecting device is so constituted that when the power source is turned on, the capacitor C1 is charged through a resistance R3 and while the terminal voltage of the capacitor C1 is low, the capacitor C1 is charged through a diode D1, etc. In this case, a light emitting diode LED1 illuminates momentarily and then turns off soon after a light emitting diode LED2 turns off. The terminal voltage across the capacitor C is held constant thereafter and the initial value of a bias voltage to a time constant circuit is set. When this bias voltage is set, an output pulse with extremely small width is obtained from a time difference detecting circuit 13, so when the back member is detected, a pulse with extremely wide width is obtained from the time difference detecting circuit 13 soon and detecting operation with high sensitivity is performed.