To obtain a sufficient gas quantity, deviate a blow-off air flow so as to rectilinearly advance against a suction air flow, and enlarge the directional range of the suction air flow, by providing a projection on a front guider and a curved part curving outside the blow-off air flow passage on the extreme end part of a rear guider.

On the extreme end part of the front guider 106 in a blow-off air flow passage 4 formed by a rear guider 107 and a front guider 106, a projection 6 projecting into the blow-off air flow passage 4, and on the extreme end part of the rear guider 107 forming the blow-off air flow passage 4, a curved part 7 curving outside the blow-off air flow passage 4, so that the angle formed by a suction air flow and a blow-off air flow 112 is generated to be nearly rectilinear. When the blow-off air flow 112 is guided with the front guider 106 and the rear guider 107 and reached to the projection 6 provided in the blow-off air flow passage 4, the projection 6 becomes an obstacle to the blow-off air flow 112, the blow-off air flow 112 is separated from the front guider 106, stuck to the A part of the curved part 7 by a Coanda effect, and blown off along the curved part 7, and hence the blow-off air flow 112 is deviated.