PURPOSE: To secure sufficient airtightness without using a sealing or coating material by providing a valve element pressing means where the line of action of the force applied to the valve element is inclined to a valve seat.

CONSTITUTION: An opening/closing rod 8 is elevated by driving a cylinder. A lock bar interfering ring 14 is turned upward by a push pin 10 through the elevation of the opening/closing rod 8, and a pressing projecting part of the lock bar 15 is abutted on a projection 6. In addition, when the opening/closing rod 8 is elevated, the valve element 5 is pressed to the valve seat, and a substrate carrying hole is closed. The valve seat is inclined downward, and when the valve element 5 is pushed upward through the rod 8, the line of action of the force by the cylinder is inclined to the valve seat to generate the wedge effect between the valve seat and the valve element 5, and the pressing projecting part works so as to be landed over the projection 6 of the valve element 5 to achieve further increase in the pressing force, and the sealability between the valve element 5 and the valve seat is increased.