There are output pistons 18 for opening intake valves 11, an intake cam 19 coaxial with a crankshaft 7 to actuate an input piston 22, a hydraulic control valve 20, output passageways 25A, 25B connecting the output pistons 18 to the hydraulic control valve, and an input passageway 24 connecting an input hydraulic chamber 21 to the hydraulic control valve. The hydraulic control valve has one state, which enables pressure-transmission communication between the input passageway and the output passageway 25A at least during a period for one cycle of reciprocating motion of the input piston 22, and another state, which enables pressure-transmission communication between the input passageway and the output passageway 25B at least during the period for one cycle of reciprocating motion of the input piston 22. The hydraulic control valve assumes the states alternately in every rotation of the intake cam 19.