An engine in an outboard motor includes a first rotation angle sensing device arranged to sense a rotation angle of a crankshaft and second rotation angle sensing device arranged to sense a rotation angle of an exhaust camshaft. A reverse rotation determination section is arranged to determine an occurrence of a reverse rotation of the crankshaft based on of a rotation angle sensed by the first rotation angle sensing device and a rotation angle sensed by the second rotation angle sensing device. A gear mechanism operation section forceaby shifts a shift gear mechanism into a neutral state when a reverse rotation of the crankshaft is sensed by the reverse rotation determination section. Water is surely prevented from entering the internal combustion engine and can minimize an increase in the manufacturing cost and simplify the maintenance work of the internal combustion engine.