PURPOSE: To readily detect a poor vacuum simply by performing an opening and closing operation of a vacuum opening and closing device, by a method wherein a poor vacuum, produced due to a vacuum leak from a vacuum valve, is detected by grasping a change in motion of a movable shaft due to a change in inner pressure of a vacuum valve.

CONSTITUTION: A movable shaft 13 of a vacuum valve 11 for a poor vacuum detector is linked to a connection metal 6 through an insulating rod 14 and a press spring 15. In case the degree of vacuum of the vacuum valve 11 is sound, a differential pressure FV, pushing upward the movable shft 13, is normally exerted on the movable shaft 13 because of a difference in pressure between an atmospheric pressure and a vacuum pressure. If a trouble occurs to the vacuum valve 11 and the inner pressure of the vacuum valve 11 is brought to the same level as that of an atmospheric pressure, no differential pressure FV is exerted on the movable shaft 13 of the vacuum valve 11. Thus, the detection of the change in motion of the movable shaft permits the easy detection of a poor vacuum simply by performing an opening and closing operation of a vacuum opening and closing device.