PURPOSE: To prevent generation of chattering in a main relay by providing a diode to allow current from a starter terminal side to an ignition terminal side between the ignition terminal and the starter terminal.
CONSTITUTION: When the key contact 20 of an ignition key 10 is contacted with an ignition terminal 26, a relay coil 32 is excited, electric power is supplied from a battery 4, and a control part 44 and the like are started to control an engine. Nextly, when the contact 20 is contacted with a starter terminal 28, a starter is actuated to start the engine. Because a diode 54 to allow current from the starter terminal 28 side to the relay coil 32 side as the ignition terminal 26 side is provided, electric power from the battery 4 can be continuously supplied to the control part 44 and the like. Hereby, the function of the control part 44 and the like is held, a fuel injection valve or the other parts can be appropriately actuated, and maloperation of the system can be prevented.