In an engine valve operating system, a timing transmission chamber (48) is formed on one side of an engine main body; the timing transmission chamber (48) houses a timing transmission system (37) that provides a connection between a crankshaft (12) and a camshaft (36) disposed above intake and exhaust valves (29i, 29e); opposite end parts of the camshaft (36) are supported by one side wall (5a) of a cylinder head (5) and a dividing wall (5b) that is adjacent to the timing transmission chamber (48); and a valve operating chamber (49) housing the camshaft (36) is defined between said one side wall (5a) and the dividing wall (5b). An oil slinger (72) is disposed in a lower part of the timing transmission chamber (48). An oil passage hole (75) is provided in the dividing wall (5b), the oil passage hole (75) guiding scattered oil that has been shaken off in an upper part of the timing transmission system (37) to the valve operating chamber (49). An oil return passage (77) is provided for transmitting pressure pulsations generated in the crank chamber (9) to the valve operating chamber (49), and making oil that has collected in the valve operating chamber (49) flow downward to the crank chamber (9) within the crankcase (2). Thus, the interiors of the timing transmission chamber and the valve operating chamber which are separated from each other can be lubricated without using an oil pump, while maintaining an arrangement in which the camshaft is disposed above the intake and exhaust valves.