PURPOSE: To prevent the deficiency in the pressure gauge, by providing the three- way electromagnetic valve connected to the outlet port at the pressure introduction section of a vacuum measuring pressure gauge to always keep the section at vacuum.

CONSTITUTION: First, the valve control signal is given to the three-way electromagnetic valve 14 to connect the pressure introduction section of the vacuum measuring pressure gauge 11 to the measuring pipe system and the vacuum state signal of the measuring pipe system 18 is given to the input and output unit 17 from the pressure gauge 11. Further, before the end of measurement, the second vacuum pump 16 is driven, the pressure of the vacuum discharge pipe system 19 is discharged to high vacuum from the upper limit of the measuring range of the pressure gauge and the vacuum is held by closing the valve 14. When the end of measurement is informed with the stop signal of the first vacuum pump 13, the unit 17 gives the selection signal to the valve 14 before the pressure of the pipe system 18 is atmospheric pressure, to conduct the section of the pressure gauge 11 and the pipe system 19, allowing to avoid the excessive pressure to the section at selection. Thus, the deficiency of characteristics of pressure gauge can be prevented.