Valve operating system for engine

A valve operating system for an engine is provided. Switching of a cam element, for closing and opening a valve, of a cam mechanism can be reliably conducted, and the lift part height of an end cam is reduced. First and second lock mechanisms (45, 46) are configured in a way that when the cam mechanism moves from a second specified position toward one side of the direction of a cam shaft and a first press part (43a) crosses a first top (55), a second press part (43b) is moving on a second top (56), and the first press part presses an inclined portion (51a) of a first groove (51), thus facilitating movement of the cam mechanism toward above one side; and when the cam mechanism moves from a first specified position toward the other side of the direction of the cam shaft and the second press part crosses the second top, the first press part is moving on the first top, and the second press part presses an inclined portion (53a) of a third groove (53), thus facilitating movement of the cam mechanism toward the other side.