To measure the evacuated pressure of an insulated vacuum tank and evacuate the tank under such a condition that a rotor is not completely stopped, but rotated at a low speed by turning, etc., by installing a gate valve to the evacuating route on an evacuating pump side from a vacuum pressure gauge.

When the rotor 1 of a superconducting rotary electric machine is rotated at a low speed, a vacuum chamber 38 is evacuated, and after it is confirmed that the inside of the chamber 398 has been evacuated to a sufficiently low vacuum pressure with a vacuum pressure gauge 19, an electromagnetic gate valve 16 immediately in front of an evacuating pump 17 is closed. After it has been confirmed that the vacuum pressure in the chamber 38 can be maintained, a plug 31 is sealed with a plug-holding tool 42, by actuating a plug-holding tool driving mechanism 43 in the chamber 38. Then the vacuum pressure of the isolated vacuum layer of the rotor 1 can be measured by communicating the evacuating route 6 of the rotor 1 with the evacuating routes 13 and 14 of an evacuating device.