of the upper end portion of the valve stem 7 and adapted to be engaged with the upper end portion of the valve stem 7 during the normal operation of the cylinder i e when the cylinder is in the operative state as shown in fig1 the cam 3 is adapted to depress the body 1 through a plate 11 fixed to the upper surface of the body 1 and held in contact with the cam 3 a snap ring 12 prevents the plunger 2 from coming out of the bore 1a in the body 1 by the force of the spring 8 a reference numeral 13 designates a groove which is formed in the inner peripheral surface of the bore 10a in block 10 and disposed in communication with the oil passage 14 while a numeral 15 denotes a computer which controls the state of the solenoid valve 5 and hence to change over the state of the cylinder a reference numeral 18 denotes a retainer which is fixed to the valve stem 7 through a cotter 19 while a numeral 20 denotes another retainer seated on the bottom of the bore 10a in the block 10 a valve spring 21 acts between these two retainers 18 and 20 on the other hand a body return spring 22 acts between the retainer 20 and the body 1 so as to urge the body 1 into continuous contact with the cam 3 a numeral 23 denotes a valve guide 23 while 24 designates a valve seat which is adapted to be contacted by the valve 4 the first embodiment is characterized by the provision of an oil draining hole 25 which is formed in a lower plate 27 constituting the bottom of the portion.