A valve mechanism for an internal combustion engine includes a first rocker arm (4) connected to an intake or exhaust valve and supported on a rocker shaft (3a), a second rocker arm (5) supported on the rocker shaft for being rotationally driven by a cam, a cylinder (10) formed on one of the first and second rocker arms (4, 5) and a first piston (11) provided in the cylinder, a contacting projection (4a) projecting on the other one of the first and second rocker arms, a return spring (12) for biasing the first piston in a direction in which the first piston contacts with the contacting projection, and a second piston (14) for displacing the first piston to a position at which the first piston does not contact with the contacting projection. The two pistons extend in parallel to each other when the first piston is at a non-contacting position.