An L-shaped rotary valve has a pair of side plates and a valve plate. The rotary valve is rotatably supported in a housing at a shaft 5. The valve plate moves along downstream ends of side wall portions of an air duct. A gap is formed between a downstream end of an air-intake passage and the valve plate, when the rotary valve is moved to its valve closed position. The side plates of the rotary valve cover the gap in a lateral direction in parallel to an axial direction of the shaft, so that leakage of air flow in the lateral direction of the gap is suppressed.