To enable a cylindrical vessel for discharging hydrogen made of an alloy of palladium and a silver more rigid than a vessel made of palladium to be reliably mounted in an airtight state despite of a kind of a metal for mounting.

A packing 10B consists of a cylinder part 12 made of a material equivalent to or softener than at least one of a cylindrical vessel 20 and a mouthpiece 30 and having an outside diameter slightly smaller than the inside diameter of a parallel part 24 of the cylindrical vessel 20; and a taper-form part 11 expanding in a divergent shape from one end of the cylinder part 12. A packing 10A having no cylinder part 12 but consisting of only the taper-form part 11 is acceptable.