PURPOSE: To smoothly select a cylinder number, by opening and closing openable valves provided in air intake passages on a rest side of a cylinder number controlled engine at an operating speed according to a rate of change in load.

CONSTITUTION: Openable valves D are provided in air intake passages C to cylinders on a rest side of a cylinder number controlled engine A. A throttle valve is provided with a throttle position sensor as a load detection means I. A rate of change in load is computed from a throttle valve operating speed by a load change rate computing means J, and an operating speed of the openable valves according to the rate of change is computed by a control means K. A cylinder number is decided according to an operating condition as detected by an operating condition detection means F, and the openable valves D are opened and closed at a predetermined speed according to a signal from the control means K thereby to select the cylinder number.