PURPOSE: To evacuate a vessel to an ultrahigh vacuum with a simple device at a low cost by using plural elastomer seals by firstly compressing only the seal on the atmosphere side to evacuate the vessel and then compressing the seal on the vacuum side to evacuate the vessel.

CONSTITUTION: Two grooves 31 (depth 5) and 32 (depth 3) are provided in a flange 23, an elastomer seal 24 is placed in the groove 31 and an elastomer seal 25 smaller than the former in the groove 32. Only the seal 24 on the atmosphere side is firstly compressed to evacuate a region 30 by a vacuum pump 26, the seal 25 on the vacuum side is then compressed after a while, and the space between the seals 24 and 25 is evacuated. At that time, since the space between the seals 24 and 25 is three places of magnitude lower than atmospheric pressure, the amt. of gas to be permeated through the seal 25 into the region 30 is reduced. Consequently, the pressure in the region 30 in the vessel is lowered as compared with the case when the seal is used in the conventional manner.