Engine valve operation mechanism

In an engine valve operation mechanism, opposite ends of a support shaft that supports a cam in a rotatable manner are supported in a first support hole of a first support wall formed in a cylinder head and a second support hole of a second support wall placed inwardly from the first support wall relative to the cylinder head, thereby allowing the support shaft to be inserted into the first support hole and the second support hole from outside the cylinder head, and an inner face of a head cover prevents the support shaft from becoming detached from the first support hole. The axial movement of the cam support shaft can thereby be restricted without employing a special stopper member, and the leakage of lubricating oil for the support shaft can be prevented without using a special sealing member.