PURPOSE: To generate a high vacuum in a short time in a main chamber closed under the atmospheric pressure.

CONSTITUTION: A plurality of auxiliary chambers 3, 4, and 5 are connected in juxtaposition on the suction side of a vacuum pump 2. The auxiliary chambers 3, 4, and 5 connected to a main chamber 1, being an object brought into a vacuum, through communicating pipes 30, 40, and 50, respectively, and on-off cocks 3a, 4a, and 5a are arranged in the middle of the communicating pipes 30, 40, and 50, respectively. The interior of each of the auxiliary chambers 3, 4, and 5 is exhausted with the aid of a vacuum pump and maintained at a constantly high vacuum. When a high vacuum is generated in the main chamber 1 closed under an atmospheric pressure, on-off cocks 3a, 4a, and 5a are released, in order, and an internal pressure in the main chamber 1 is reduced in stages through communication of it with the auxiliary chambers 3, 4, and 5.