Valve operation control system for internal combustion engine

A valve operation control system for an internal combustion engine comprising a hydraulic valve operation mode changing mechanism (63, 75) for switching the operation mode of an engine valve (VI, VE) openably and closably disposed in a cylinder head (22R), and hydraulic pressure control means (108R) for controlling hydraulic pressure that is applied to the valve operation mode changing mechanism (63, 75), the hydraulic pressure control means (108R) comprising a holder (109) mounted on the cylinder head (22R), a spool valve (111, 112) formed by slidably housing a spool valve body (123, 125) in a valve body (110) provided so as to be connected to the holder (109), the spool valve body (123, 125) having one end part facing a pilot hydraulic chamber (122, 124), and an electromagnetic open/close valve (113, 114) for controlling hydraulic pressure of the pilot hydraulic chamber (122, 124), characterized in that the holder (109) is formed by integrally connecting a housed portion (109a) and a projecting portion (109b) by means of a connection portion (109c), the housed portion (109a) being housed between the cylinder head (22R) and a head cover (23R) secured to the cylinder head (22R), the projecting portion (109b) projecting outside the cylinder head (22R) and the head cover (23R) and having the electromagnetic open/close valve (113, 114) mounted thereon, a seal face (116, 117) between the head cover (23R) and the cylinder head (22R) is formed on the connection portion (109c), and the valve body (110) is provided so as to be connected to the housed portion (109a) while being housed between the head cover (23R) and the cylinder head (22R).