PURPOSE: To obtain a holding circuit transmitting an output signal not including chattering and suitable for a condition signal to a power supply circuit by using a photocoupler.

CONSTITUTION: When a detecting contact ALMa is closed, a light emitting diode of the photocoupler PIa is driven and lighted, and a light emitting diode LED for display is lighted. The luminous flux of the light emitting diode of the photocoupler PIa is incident to a photodetecting transistor (TR), a collector of the photodetecting TR goes from logical "1" to logical "0", an internal resistor is lowered attended with the conduction of itself so as to form the holding circuit. When the photodetecting TR is turned on, a smooth signal without chattering is obtained at the collector output of the photodetecting TR even if chattering exists in the detecting contact ALMa, and outputted via an inverter INV and an NOR circuit NOR.