PURPOSE: To reduce a power loss upon normal operation by a method wherein a reverse flow preventing circuit, having a reverse polarity with respect to a DC power source, is interposed in the current supplying line of a driving coil for a relay, having an normal close contact interposed in a power supplying line for a load from the DC power source.

CONSTITUTION: A positive terminal 12a of a DC power source 11 is connected to the terminal 14a of a load or a discharging lamp device 14, for example, through the normal close contact 13a of a relay. The negative terminal 12b of the DC power source 11 is connected to the terminal 14b of the load. The coil 13b of a relay, driving the normal close contact 13a, is connected in series to a diode 15 employed as a circuit for preventing reverse flow to connect them between power source terminals 12, 12b while the diode 15 is connected in reverse polarity with respect to a storage battery 11. When the load 14 is connected to the storage battery in a correct polarity, a power is not consumed substantially but the coil 13b is conducted and the normal close contact 13a is opened when the polarity of the storage battery 11 is wrong. According to this method, erroneous connection may be prevent with a small power consumption. This device is optimum for a storage battery circuit for vehicles.