PURPOSE: To provide practicability of easy fitting of a seal ring to a ring-shaped dovetail of a valve seal part by making a ring from crystalline plastics, and embodying the ring in the form which ensures tight fitting.

CONSTITUTION: A seal ring 1 is made of polyester resin through molding process. The resultant is subjected to a molding process, cooled rapidly, and fixed to a form of a ring 2. This ring 2 is put in a dovetail 4 of a packing seal part 3, heated with the steam in an enclosed trough, and fitted in the internal surfaces 5 of the dovetail 4 so as to be tightly contacting the external surface of the seal ring 1. This enables easy fitting of the seal ring to prevent transfer of odor to the fluid on the line. Thus contamination can be precluded.