To prevent excessive abrasion on a part with which a seal or a sealing surface makes contact by forming a cylindrical recessed part respectively at least on a part of a plural number of protruded parts and by vertically orienting each of the cylindrical recessed parts with a back surface of the seal.

An installation ring is arranged in a cavity 66 of a first divided part 62 of a metal mode 60 while making a seal assembly body, and it is sealed and made contact with an adjacent flat surface 70 of the first metal mold divided part by moving a flat surface 72 adjacent to a second metal mold divided part 64. An elastic material of a seal is injected into a closed chamber, an irregular seal is formed, and this seal is connected to the installation ring. The first and second metal mold divided parts 62, 64 are separated from each other, and the completed seal assembly body is removed. As a cylindrical protruded part 86 extending from a recessed part 84 of the second metal mold divided part is directed in the vertical direction against a divided line 74, the cylindrical protruded part is easily removed from the recessed part of the irregular seal.