Abstract: An engine comprising a main intake passage having a throttle valve therein. A bypass passage is branched off from the main intake passage located upstream of the throttle valve and is connected to the main intake passage located downstream of the throttle valve. A flow control valve, actuated by a step motor, is arranged in the bypass passage. When the engine is operating in an idling state, the step motor is rotated at a first speed in a rotating direction wherein the engine speed approaches a desired engine speed. When the ignition switch is turned to the OFF position for stopping the engine, the step motor is rotated at a second speed which is lower than the first speed a little while after the flow control valve fully opens the bypass passage.