PURPOSE: To facilitate the mounting of a mouth member by a method wherein the mouth member with an outer diameter thereof smaller slightly than the inner diameter of an internal member is fitted into a terminal part of the internal member of a metal vacuum bottle, a terminal part of an external member with an inner diameter thereof smaller slightly than the outer diameter of the internal member is fitted forcibly into the terminal part of the internal member and the terminals of the internal and external members are welded together.

CONSTITUTION: A mouth member 7 with an outer diameter thereof smaller by a slight value (a) than the inner diameter of a terminal part 8 is fitted into the terminal part 8 of an internal member 2 of a metal vacuum bottle which is made vacuum between the internal member 2 and an external member 3. The inner diameter of an upper end part 9 of the external member 3 is made smaller by a value (b) than the outer diameter of the terminal part 8 of the internal member 2 to fit the internal member 2 forcibly into the external member 3. The value (b) is set larger slightly than the (a). Terminal parts 9 and 8 of the external member 3 and the internal member 2 are welded 10. The terminal part 8 of the internal member 2 is reduced diametrically to tighten the mouth member 7. This facilitates the welding of the internal member 2 and the mouth member 7 under a pressure.