PURPOSE: To obtain a small-sized, inexpensive electric power unit by providing a capacitor for backup and for a filter of a DC-DC converter, providing a switching means between the capacitor and a load except an ignition system so as to prevent generation of electric supply from the capacitor to the load at the time of stopping output from a battery.

CONSTITUTION: In a device equipped with G sensors 4, 5 detecting collision of automobiles to let ignition current flow to a starting means 3 and making an air bag develop while equipped with a boosting DC-DC converter, a capacitor 17 for a filter connected between the output side of the DC-DC converter 8 and the earth is provided while a switching transistor 18 for connecting and disconnecting the capacitor 17 and a load 10 is provided. An electric power supply condition detecting transistor 14 is provided. When electric power from a battery 1 is cut off, the transistors 14, 18 are made to OFF state, charging voltage for the capacitor 17 is maintained at a specified voltage even if output of the DC-DC converter 8 is cut off, and sufficient ignition current for the starting means 3 can be supplied at the time of closing the G sensors 4, 5.