PURPOSE: To improve bending strength of a screw thread part by arranging the fibrous reinforcing material such as a glass fiber and the like radially in shaft diameter direction of a bolt/nut, and covering the bolt/nut with the fibrous reinforcing material.

CONSTITUTION: A bolt/nut is covered with fibrous reinforcing materials 4, 15 which are arranged radially in shaft diameter direction of the bolt/nut. When a bolt is manufactured, the center part of a fibrous reinforcing material bundle 4 having a nearly equal length to the outer diameter of a screw thread 1b part is wound around respective twist rolling positions while twisting a plurality of wire rods 3 so as to manufacture a reinforcing core material 2 provided with the shape of a bottle washing brush, the reinforcing core material 2 is concentrically arranged in the forming mold of a bolt so as to fill a plastic material. The reinforcing core material 12 for a nut 11 may be constituted by implanting the short fibrous reinforcing material 15 in a raising blanket shape along one surface of a sheet shaped base material 13.