To prevent the functional stop of an engine start section caused by the failure of a relay by providing a plurality of relays connected in parallel with the signal line connecting a battery and the engine start section, and switching another relay to an on-state when no voltage of the prescribed level is detected in the on-state of one relay.

A switch 4a is turned on after a driver operates an ignition key 1 to a start position, and a first relay 5a is turned on by a controller 8 when a voltage of the prescribed level is measured by a first voltmeter 6. Power is fed to an engine start section 3 from a battery 2 as a result, and an engine is started. If the first relay 5a fails and no voltage of the prescribed level is measured by a second voltmeter 7 at this time, the controller 8 outputs the control signal for setting a second relay 5b to an on-state in place of the control signal for setting the first relay 5a to the on-state, thereby the second relay 5b is turned on to drive the engine start section 3.