A blowoff orifice is constituted by an upper wall, a lower wall and a vertical wind deflecting plate. The upper wall inclines so that a flow passage becomes narrow toward downstream and provided with a protrusion at its end portion. The lower wall has a horizontal linear portion on the downstream side and an end portion forming an acute angle at the tip of the linear portion. The vertical wind deflecting plate is provided between the upper wall and the lower wall, and capable of changing an airflow from a horizontal direction to a downward direction. The upper wall protrusion is located more downstream than the lower wall end portion.