PURPOSE: To prevent the generation of a noise by a fluctuation of discharge pressure when suction pressure reaches the saturated state.

CONSTITUTION: A vacuum pump 1 has a first suction opening 2a having a check valve 8 and a second suction opening 2b opened and closed by a solenoid valve 7. When the negative pressure in a vacuum tank 4 is larger than a determined value, the solenoid valve 7 is closed by a pressure switch 6, and the air in the vacuum tank 4 is sucked through the first suction opening 2a. When the negative pressure in the vacuum tank 4 is smaller than the determined value, the solenoid valve 7 is opened by the pressure switch 6, and the atmosphere is sucked through the second suction opening 2b. Even when the suction pressure of the first suction opening 2a gets close to the saturated state, thus, the internal pressure in a pump chamber 13 is increased to suppress a fluctuation of discharge pressure, and generation of a noise can be prevented.