To eliminate defective operations of an ECU, and reduce cost of the ECU and miniaturize the unit.

The power circuit of an outboard motor is provided with an engine control unit 14 for controlling the operations of an engine, a battery 11 connected to the unit via a relay device 79, and a main switch 9 for performing self-holding or release of the relay device. When the main switch is turned on, the self-holding of the relay device is performed by a main switch signal or an engine control signal, and when the main switch is turned off, the self-holding of the relay device in which the engine control signal is turned off, is released after a prescribed time has elapsed.