Abstract: There are provided a casing (21), a valve element (23) that effects opening/closure of an evacuation port (22), and means for driving (24) that effect opening/closure drive of this valve element. Fine irregularities (23b) are formed on valve element (23). The means for driving comprise: a cylindrical yoke (25); a core (26) provided with a cylindrical part (26a) arranged within the yoke and a flange (26b) that is abutted by one end of the yoke; a magnet (27) arranged within the yoke and that forms magnetic flux in the radial direction; a movable element (28) arranged through the cylindrical part of the core with the valve element mounted at one end; a coil (29) arranged in the gap between the magnet and yoke and fixed at one end of the movable element; and holding members (30) that hold both ends of the movable element and that hold the movable element and coil in non-contacting condition with respect to the core and yoke.