PURPOSE: To reduce the fluctuation of torque, by stopping operation of two cylinders, the pistons of which are operated at the same phase with each other, and opening the suction valves for a prescribed while at the position near the bottom dead point at the time of partial-cylinder operation of a four-cylinder internal combustion engine.

CONSTITUTION: When the engine load detected by a control circuit 17 of a four- cylinder internal combustion engine is low, the operation of intake valves and exhaust valves of two cylinders #1, #4 the pistons of which are operated at the same phase is switched by means of an actuator 10, and the ignition is stopped by opening switches 19A, 20A provided respectively at an intermediate portion of ignition cords 19, 20, thereby starting partial-cylinder operation. In this state, the intake valves are opened for a period from the time corresponding to the position 30° before the bottom dead point of the suction stroke to the time corresponding to the time 60° after the bottom dead point. Therefore, even if compression gas is blown off to the crank case during the rising stroke of piston of inoperative cylinders, a proper compression pressure is obtained always by supplying fresh air in the next cycle, so that it is enabled to reduce the fluctuation of the torque and engine speed.