PURPOSE: To calibrate the vacuum gauge accurately in an extremely high vacuum state by providing a shield plate for prevention against discharge to a vacuum gauge fitting part, and providing an orifice which has small conductance between a gas supply part and the vacuum gauge fitting part.

CONSTITUTION: A test dome 2 is partitioned into two chambers with the orifice 3 and a calibrating vacuum gauge 4, numbers of vacuum gauges 6 to be calibrated, and a gas analyzer 5 are fitted in the downstream-side chamber to which a vacuum pump 1 for evacuation is connected. The shield plate 7 for prevention against discharge is arranged among the respective vacuum gauges. Then, a vacuum gauge 8, gas supply circuit 9, and auxiliary evacuating circuit 10 are connected to the upstream-side chamber.