PURPOSE: To maintain the accuracy of measurement and save the labor for disassembling by rushing air into a vacuum device through a small sized air lead-in tube provided near the gage port of a gage mounting and connecting pipe connected to a vacuum valve communicating with said vacuum device thereby maintaining a gage terminal clean.

CONSTITUTION: A vacuum device 1 is connected to a gage port 9 through a conduit 2 for measuring the degree of vacuum, a vacuum valve 3 and a pipe 4 for connecting a gage. A terminal 10 for measuring the degree of vacuum is provided in the port 9. A nozzle 8 having ejector effect is provided near the port 9, and rush-in of air is made possible by a small sized air lead-in valve 7. At the time of putting the inside of the device 1 under atmospheric pressure, the valve 7 is opened for a short time prior to opening of a large sized air lead-in valve 12 to allow air to rush in through the nozzle 8, thereby blowing the dust stuck and accumulated near the port 9 toward the device 1 and cleaning the same. After this, the valve 3 is closed and the valve 12 is opened. Thereby, the accuracy of measurement is maintained without disassembling and cleaning the device every time.