Valve mechanism for internal combustion engine

A valve mechanism for an internal combustion engine, comprising:

• a first rocker arm whose end is connected to either an intake valve or an exhaust valve, said first rocker arm being supported by a rocker shaft in an oscillatory manner and being driven by a first cam;
• a second rocker arm provided adjacently to said first rocker arm, said second rocker arm being supported by the rocker shaft in an oscillatory manner and being driven by a second cam whose shape is different from that of the first cam;
• a cylinder provided for either said first rocker arm or said second rocker arm, said cylinder communicating with an oil passage, which is formed through said rocker shaft, by way of a communicating path formed through the wall of the oil passage;
• a first piston fitted in said cylinder in a slidable manner;
• an engaging protrusion provided in a protruding condition for the remaining one of said first rocker arm and said second rocker arm, said engaging protrusion being capable of engaging with an engaging part of said first piston;
• a return spring which applies a force to said first piston for impelling said first piston to a position where said first piston does not engage with said engaging protrusion;
• and
• a second piston which is moved by hydraulic oil supplied from the oil passage so as to move said first piston to an engaging position where said first piston engages with said engaging protrusion, said movement being made against the force applied by said return spring.