PURPOSE: To allow normal voltage to be applied to the load even if a positive electrode and a negative electrode are reversely connected by connecting a diode between a terminal for power supply and an internal circuit, on an electronic device which is driven from a dc power supply.

CONSTITUTION: A positive electrode and negative electrode of an external dc power supply box 6 are connected to a first terminal for power supply 1 and a second terminal for power supply 2 respectively. Current flowing from the positive electrode of the external dc power supply box 6 reaches a positive electrode of the load 4 through a diode 30 and then passes through a negative electrode and returns to the negative electrode of the external dc power supply box 6 through a diode 3'. Nextly, the negative electrode and positive electrode of the external dc power supply box 6 are connected to the first terminal for power supply 1 and the second terminal for power supply 2 respectively. In this case, too, current flows through the diode 30 and then the diode 3' and current is normally supplied to the load.