PURPOSE: To enhance the rigidity of a rocker arm in a variable valve timing mechanism.

CONSTITUTION: The proximal end part 9'c of a piston 9' for coupling a rocker arm 5 and a locker arm 7 with each other, has a larger diameter, and is slidably fitted in a large diameter part 4c of a piston hole 4b formed in a rocker shaft 4, and during the coupling, opposite end parts of the piston 9' are supported by the piston hole 4b and the large diameter part 4c so as to reduce bending due to the valve load, thereby it is possible to enhance the rigidity of the rocker arm 7.