In a cylinder head cooling structure of engine E including a camshaft 15 rotated by a crankshaft 2, a push rod, and an actuatable rocker arm, an injection hole 46 for injecting oil pressurized by an oil pump 44 is formed in a shaft portion 43 between an exhaust cam 36 on the camshaft 15 and a journal 15j at its rear end, and the oil injected the injection hole 46 is allowed to enter a rear cylinder head cover 7 through a pushrod chamber in which the push rod is disposed.