To prevent the set position of a piston from being changed so that a stroke shortage is never caused by setting the volume of a sealing material larger than the capacity of an insert groove, providing a protruding clearance for the sealing material on the inner side from the insert groove of them fitting surface between an upper case and a lower case, and fixing the upper case to the lower case by caulking.

A case body 1 is divided into an upper case 2 and a lower case 3, an insert groove for an O-ring 4 of sealing material, or an O-ring groove 11 is formed on the fitting surface between the upper case 2 and the lower case 3. The volume of the O-ring 4 is set larger than the capacity of the O-ring groove 11, a clearance α for protruding is provided on the inner side from the O-ring groove 11 of the fitting surface A-A between the upper case 2 and the lower case 3, and the upper case 2 and the lower case 3 are fixed together by caulking. Thus, since an expansion body 6 can not be protruded into the O-ring groove 11, the capacity of an expansion body chamber 5 is constant, so that the set position of a piston is never changed, and a stroke shortage is thus never caused.