In a scooter engine having valve lifters (33) interposed between intake valves (16) and a cam shaft (31), as well as between exhaust valves (17) and a cam shaft (32), with its axial line (C) of a cylinder (7) extending toward the front part of the vehicle body, a stopper (42) is provided in the cylinder (7) in the opposite position to a top face (33a) of the valve lifters (33). The top face (33a) of the valve lifters (33) is abutted against the stopper (42) when the valve lifters (33) move to fall off a cylinder head (12) as the cam shaft is removed from the cylinder head (12), thus the valve lifters (33) are prevented from falling off the cylinder (7) by the stopper (42). In this way, the valve lifters (33) are be kept assembled to the cylinder (7) even when the cam shaft is removed during the maintenance work. In other words, the DOHC engine (2) can be installed on a scooter while preventing the valve lifters (33) from falling off in the course of maintenance work.