A valve operating mechanism for an internal combustion engine having cams rotatable in synchronism with a crankshaft, a pair of valves disposed in intake or exhaust ports of a combustion chamber, and four pivotable rocker arms for transmitting the lifting motion of said cams to said valves. Four cams are provided with different cam profiles corresponding to respective ranges of rotational speeds of the engine. Switching means are operated by hydraulic oil pressure for selectively connecting and disconnecting the rocker arms to thereby operate the valves in several different modes for different engine speeds.