circular flange 20a which extends from the outer circumference of the bellows 19 the flange 20a has holes through which bolts 21 protruding from the base plate 12 are loosely inserted each of these bolts 21 penetrates a spacer 22 interposed between the base plate 12 and the flange 20a to keep the space between them fixed a nut 23 is screwed on the tip of the bolt 21 protruding from the flange 20a interposed between the nut 23 and the flange 20a is a compression spring 24 surrounding the bolt 21 whereby the contact plate 20 is urged toward the base plate 12 a discoid projection 12a is formed on the inner face of the base plate 12 so as to be located inside the bellows 19 the dimensions of the projection 12a are so determined that there may be given a circular space between its circumferential surface and the inner surface of the bellows 19 and between its end face and the inner surface of the contact plate 20 sealed in the container 18 is a working fluid with a relatively high vapor pressure such as freon 114 trade name in fig2 numeral 25 designates a heat insulating material which.