A durable vacuum opening/closing valve connectable with a vacuum chamber and a vacuum pump is arranged to control vacuum pressure in the vacuum chamber by changing an opening degree of an O ring serving as a valve element relative to a valve seat. A predetermined clearance between an outer peripheral surface of a piston and an inner peripheral surface of a cylinder is sealed by a bellofram which changes its shape in association with movement of the piston moved to open or close the valve. The bellofram includes an inclined surface with a predetermined inclination angle and the outer periphery of the piston includes an inclined surface with a predetermined inclination angle, so that an inner diameter of the bellofram in contact with the piston and the outer diameter of the piston are equal when the valve element is in contact with the valve seat.