PURPOSE: To implement the miniaturization and easier maintenance of the captioned protecting circuit adaptable to a small power equipment.

CONSTITUTION: When a direct current power source is correctly connected to terminals 1 and 2 on the input side, the direct current flows to a relay 8 through a diode 9 if a switch 10 is closed. When the direct current flows, the relay 8 causes contacts 7a and 7b to be closed, thereby to supply the direct current to terminals 3 and 4 on the output side. On the other hand, when the direct current power source is reversely connected to the terminals 1 and 2 on the input side, the direct current does not flow even if the switch 10 is closed because its flow is blocked by the diode 9. The contacts 7a and 7b are not closed, either, thus allowing no direct current to be supplied to the terminals 3 and 4 on the output side. Therefore, it is possible to prevent the circuits on the equipment using the direct current power source from being destroyed by the application of any inverse voltage.