A valve drive system of an engine includes a camshaft that is supported by a cylinder head of the engine and on which a plurality of cams having different valve lift characteristics are arranged at predetermined intervals, a rocker shaft that is supported by the cylinder head so as to be parallel or substantially parallel to the camshaft, and a rocker arm that is swingably supported by the rocker shaft. The rocker arm is arranged between one of the cams and an intake valve or an exhaust valve and is arranged so as to be movable in an axial direction of the rocker shaft. A presser of the rocker arm that presses the intake valve or the exhaust valve extends in the axial direction and has a length greater than an interval between the cams. The valve drive system includes a drive unit that moves the rocker arm toward one side or toward an opposite side in the axial direction by the interval between the cams.