A power source system for a small boat is equipped with a main switch with a lanyard switch capable of turning OFF power source in case of an emergency by connecting to an occupant by wire. The power system controls the power source to accessories, including a fuel injection system. The power source system includes a main relay for turning ON/OFF power source to be supplied to the accessories, and a control unit (display control unit) connected in parallel to the main switch in order to control the main relay. An ON/OFF state of the main switch is monitored by the control unit (display control unit) to control the ON/OFF state of the main relay on the basis of the ON/OFF state of the main switch. With this system, it is possible to collectively control the ON/OFF of the power source supplied to the accessories, including the fuel injection system. Thus, the power source system can be simplified.