An engine adjusts the amount of intake air by cooperative control for the valve duration of the intake valve and the opening degree of the throttle valve. In the engine, the valve duration is fixed to a warm-up valve duration .theta.w in a low-to-middle speed range of the engine during a period from immediately after starting of the engine to when warm-up is completed. In a high speed range, on the other hand, the valve duration is increased from the warm-up valve duration .theta.w in accordance with increase in the engine speed Ne.