To prevent wasting of battery voltage by introducing a detection signal from a noise detection circuit to detect ignition noise generated in accordance with engine starting and blocking voltage supply to the side of a load when this detection signal is not input.

Since generation of ignition noise is stopped when a driver turns off a key by stopping a vehicle after travelling the vehicle, voltage of an L-level is not input to a control part 5. This control part 5 starts time counting of a timer at the point of time when a detection signal of noise is not input and actuates a switching circuit 4a of an electric power source part 4 to turn off when two minutes pass. Consequently, battery voltage is not supplied to the side of the load, and wasting of the battery voltage is stopped in a short period of time after an engine is stopped.