PURPOSE: To reduce leakage current at the off-time of a proximity switch and to reduce remaining voltage at the on-time by connecting constant current and constant voltage means to the output terminal of the proximity switch and also connecting a circuit for short-circuiting the constant current means at the on- time of the switch.

CONSTITUTION: When an object M to be detected does not approach a detecting coil l of a proximity switch, a signal is not supplied from a detecting circuit A, transistors (TRs) Q1∼Q3 are turned off and current I0 supplied from a constant current diode I is the sum of the current consumption IA of a circuit A and the current IZ flowing into a constant voltage diode Z, so that leakage current is not unnecessarily consumed. When the object M approaches the coil l, TRQ1∼Q3 are turned on by an output of the circuit A and the constant current source I is short-circuited by the TRQ1. Even if current IB3 flowing into the TRQ3 is small, the current IB3 is compensated by current IR flowing into a resistor R0 and the circuit A is stably driven. The remaining voltage at the on-time of the proximity switch is limited only the voltage VZ applied to the circuit A and the voltage applied between the base and emitter of the TRQ3 and the voltage is not unnecessarily consumed.