Abstract: A valve disabling device has an engaging member and an urging member slidably disposed, respectively, in first and second axial bores formed within, respectively, first and second rocker arm means, one of which engages a valve actuating cam. The engaging member is displaceable within the first axial bore by means of pressure applying means, into the second axial bore for driving coupling of the first and second rocker arm means whereby a pair of inlet valves or exhaust valves corresponding to the first and second rocker arm means make valving motions in unison with each other, or out of the second axial bore for disconnection of the first and second rocker arm means from each other to disable part of the valves. Biasing means biases the engaging member into the second axial bore irrespective of operation of the pressure applying means, to correspondingly bias the urging member into a predetermined position where a portion thereof is projected out of the second axial bore.