PURPOSE: To avoid the operation of a battery relay when a power supply battery is reversely connected and protect respective loads from burning damages caused by an overcurrent applied to a main wiring, etc.

CONSTITUTION: A diode 22 is provided in one side line 21B of an auxiliary wiring 21 in such a manner that its anode is connected to one side contact 16B of a key switch 16 which has a high potential in a normal state and its cathode is connected to the coil 10C of a battery relay 10 which has a low potential in a normal state. Therefore, if the key switch 16 is turned on while a power supply battery 8 is connected with a reverse polarity, a current from the power supply battery 8 is supposed to flow through the auxiliary wiring 21 to a direction shown by an arrow I1 but the current is blocked by the diode 22. With this constitution, the coil 10C of the battery relay 10 is not excited and the battery relay 10 is not closed.