PURPOSE: To aim at the promotion of smoothness in engine speed variations at the time of operating the engine concerned, by shifting the valve timing of a suction or exhaust valve so as to make any one of cylinders at the stopping side turn to a pump cylinder but the rest to a compression cylinder respectively when these cylinders are partially set in operation.

CONSTITUTION: When cylinders #1 and #4 are shifted to those at the operating side but the rest #2 and #3 to those at the stopping side, a suction passage branches off to suction passages 13 and 14 at the stopping side and the operating side respectively at the downstream of a throttle valve 12. At the upstream of the suction passage 13 at the stopping side, there is provided with a fresh air cutoff valve 15 being operated by an actuator 16. In addition, these cylinders #2 and #3 at the stopping side are turned to a compression cylinder #2 and a pump cylinder #3 each at the stopping side, and a suction valve of the cylinder #2 is closed for a little duration at the vicinity of a piston bottom dead point while an exhasut valve is made turning to a state of stoppage. Furthermore, the valve timing of a suction or exhaust valve in the cylinder #3 is controlled by an actuator 10 so as to be set up contrary to the aforesaid valve timing.