A dynamic valve mechanism for an engine which can make the overall height of the engine relatively small so as to provide easy loading of the engine on a vehicle. The dynamic valve mechanism comprises a cam shaft 35 disposed on a cylinder head 34, an exhaust cam 36 disposed on the cam shaft and drives an exhaust valve 42 through an exhaust rocker arm 40, a rocker shaft 45 serving as the center of rocking motion of the exhaust rocker arm, an exclusive cam 38 disposed side by side with the exhaust cam 36 on the cam shaft, an oil passage housing 49 disposed to pass through under the rocker shaft 45 and having its one end facing the exclusive cam 38 and the other end facing the exhaust valve 42, a master piston 50 operated by the exclusive cam 38 to supply a hydraulic pressure to the other end of the oil passage housing 49, and a slave piston 54 which, under the hydraulic pressure supplied by the operation of the master piston, opens the exhaust valve 42 at a timing different from the valve opening timing by the exhaust cam 36.