To measure an ultra high vacuum pressure accurately by varying the conductance value and gas supply amount at two minimum values, respectively, and then measuring the pressure in a vacuum vessel for measuring for at least four states (conditions) being determined by the combination of these values.

In a state (operating condition) that a nude ionization vacuum gauge 14 is lighted, the position of a movable disc 17 is controlled 21 in movement to satisfy at least two conditions (conductance) and the opening of a micro flow rate regulation valve 19 is controlled 22 to satisfy at least two conditions (gas flow rate). Under at least four different conditions determined through combination of these two conditions, the nude ionization vacuum gauge 14 measures the pressure in a vacuum measuring container 13 fixed to a vacuum vessel 2 to be measured. A computer 23 processes the pressure data obtained through control under these four conditions and determines the ultra high vacuum pressure in the vessel 2 under a state where vacuum gauge 14 is unlighted (nonoperating state).