In a vacuum pressure control system constituted of a vacuum vessel, a vacuum pump sucking gas in the vacuum vessel, a vacuum proportional opening and closing valve disposed on a pipe connecting the vacuum vessel and the vacuum pump, the vacuum proportional opening and closing valve changing its opening to change the vacuum pressure in the vacuum vessel, a pressure sensor to measure the vacuum pressure in the vacuum vessel and a vacuum pressure control device to control the opening of the vacuum proportional opening and closing valve based on the output of the pressure sensor, the vacuum proportional opening and closing valve is provided with a valve seat, a valve member with a tapered surface in its outer periphery and a pilot valve, the valve member being movable along a center line of the valve seat to change a clearance area between the valve seat and the tapered surface, and the vacuum pressure control device controls a servo valve to change the pressure of air to be supplied to the pilot valve based on the output of the pressure sensor.