To solve the problems that, heretofore, a high vacuum environment maintaining method for treating operations such as seam welding in a high vacuum environment has not been constructed, and that requirement of a treatment method in a high vacuum environment as recent new various parts production methods and an insulating function is produced, and, in an air environment, in the case an ignition phenomenon is caused, after setting to a high vacuum environment, to achieve the production of parts into which a gas preventing ignition is introduced and in which an ignition phenomenon is not caused.

In the partition structure between a high vacuum chamber 1 and an air side 2, the middle of the high vacuum chamber and the air side is provided with an area of a low vacuum chamber 5, a vacuum difference between respective sealing parts is made narrow, and the vacuum difference is made stepwise adjustable.