Sealing mechanism 6 comprises two metal bellows 7, 8 arranged one behind the other.Depending on the temperature and/or the pressure of a medium to be conducted through connection piece 1, bellows which are formed from other materials, e.g. with plastics or the like, may be used instead of metal bellows 7, 8.The metal bellows 7, 8 are provided with fastening flanges 9 to 12.