PURPOSE: To evacuate the inside of a vacuum vessel into a high vacuum in a short time by installing a degassing means, which exhaust gas left behind in a clearance part between an O-ring storage groove and an O-ring to the vacuum side, in this O-ring storage groove.

CONSTITUTION: An O-ring storage groove 13, whose section is of trapezoidal form, is installed in a flange face 12 of a flange 11 installed in the lower end of a vacuum vessel, and an O-ring 14 consisting of an elastic material is held in this groove 13. Four vent holes 15 larger in diameter than width of the groove 13 are installed on the groove 13 at four spots as a degassing means. These vent holes 15 is opened to a clearance part 16 between the groove 13 and the O-ring 14, through which gas left behind in this clearance part 16 is made so as to be exhausted to the vacuum side from these vent holes 16 in design. Now, if an opening 17 of the flange 11 is closed by a seal plate, the O-ring 14 is crushed inside the groove 13 whereby the gas left behind in the clearance part 16 is exhausted to the vacuum side from these vent holes 15 so that residual gas is in no case leaked to the vacuum side at the time of decompression, and thus it is reached to a high vacuum in a short time inside the vacuum vessel.