A multicylinder engine whose working cylinders are variably controlled in number to suit operating requirements, with an air fuel mixture fed from a single throttle valve to the working cylinder or cylinders through an intake manifold, includes at least one normally closed control valve installed in at least one intake pipe of the intake manifold. When the throttle valve has opened to a prescribed degree, the control valve instantly opens fully or almost fully or to a prescribed degree, independently of the throttle valve.