An outboard engine system includes a casing having a driving-force transmitting device provided therein, and an engine for transmitting power through the driving-force transmitting device to a propeller. The casing and the engine are carried on a hull for tilting-up movement. An engine cover made of a synthetic resin covers at least an upper portion of the engine, and an undercover comprises a pair of cover halves made of a synthetic resin and connected to each other to cover at least a lower portion of the engine from opposite sides. The undercover is detachably connected at its upper end to a lower end of the engine cover and fixed to an upper portion of the casing. In such outboard engine system, the undercover is detachably fixed at its upper portion to a body of the engine. Thus, the undercover can be supported so as to give an impression that the engine cover and the undercover are rigid during a tilting-up operation.