PURPOSE: To assure sure sealing and to reduce pressure drop in the system by press sticking a seal of an elastic material mounted to the end part of a separating tube by means of a member having high Young's modulus which is applied with permanent set.

CONSTITUTION: A sealing member 4 of an elastic material is inserted into the end part of a separating tube 1, and a member 5' which is higher in Young's modulus and smaller in elastic limit than said sealing member is mounted thereto. A metallic material particularly such as iron, steel, aluminum or stainless steel is desirable for the member 5', and it is a good practice to punch holes in a ring or to provide notches thereto in order to make the generation of permanently expanding set easy. A preferable method of mounting the member having high Young's modulus to the member 4 is to embed said member into the sealing member, because with such method there is no direct contact between the member 5' and the liquid to be treated. According to this sealing method, the sealing between the sealing member and the separating tube is effected surely, and since the narrowing of the flow passage as in the case of inserting a tubular sleeve to the member is obviated, the pressure drop is reduced.