PURPOSE: To eliminate the space between a minute hole and a fiber and minimize the eccentricity of the center of the minute hole to the outside diameter of a plug, by flowing a synthetic resin to a constituted body, where the straight fiber or wire pierces the center of a cylindrical material, and molding it and pulling out the fiber or the wire to form the minute hole.

CONSTITUTION: The straight and high-cylindricity cylindrical material which forms the outside diameter of the plug and end face forming materials which are fitted closely to this cylindrical material in its both ends are prepared, and the minute hole matched to dimensions of the fiber is provided preliminarily in the center of each of end face forming materials with a high precision. The used fiber or the wire having the same dimensions is allowed to pierce the cylindrical material and end face forming materials, and the they are assembled in this state. The synthetic resin is flowed or injected to the cylindrical material from a preliminarily provided charging entrance and is molded. Until the resin is hardened, a tensile force is applied to the fiber or the wire to prevent the slack. After the resin is hardened completely, the fiber or the wire is pulled out, and end face forming materials are removed from the cylindrical material to take out a mold product, and the plug is obtained by working as required.