To provide a structure for determining a position of a rocker arm (412,413) that allows easy maintenance, an engine (100) has a cylinder head (113) in which a valve drive mechanism (400) is provided. The valve drive mechanism (400) includes rocker arm shafts (414,415) that are fitted to the cylinder head (113); rocker arms (412,413) that are fitted to the rocker arm shafts (414,415) so as to be moveable in the axial direction; and a head cover (114) (an upper arranging member) that is attached to the cylinder head (113). The head cover (114) is provided with guide members (731,732) that engage with the rocker arms (412,413), and guide the rocker arms (412,413) to determined positions on the rocker arm shafts (414,415) when the head cover (114) is attached to the cylinder head (113).