A theft prevention apparatus of a personal watercraft equipped with a main switch for opening/closing a main power-supply circuit including a starting circuit of an engine is provided. The apparatus inhibits the engine from starting when it detects the main switch being in an ON-state, no existence of a connection between an operating device of the main switch and the main switch, and a stopping of the engine. The apparatus prevents the engine from starting by inhibiting at least one of the starting circuits of the engine, an ignition circuit of the engine, and a fuel injection circuit of the engine from closing the circuit(s).