Abstract: A natural resin is dissolved in liquid oils and fats while a phosphate-based extreme pressure additive is evenly mixed in to prepare a paint composition. After the paint composition is applied to a high-strength hexagon head bolt (2) and a hexagon nut (3), centrifugal separation is performed for removal of excess paint composition and drying of the composition. Coating films can easily and quickly be formed. The high-strength hexagon head bolt (2) and the hexagon nut (3) thus provided with a coating film are fastened to each other by screwing. The heat and pressure generated during the fastening process cause the phosphate-based extreme pressure additive to react with the metal on the surface of the high-strength hexagon head bolt (2) and the hexagon nut (3) so that the phosphate-based extreme pressure additive in the film bonds to the metal. The coefficient of friction is reduced regardless of the smoothness of the surface of the high-strength hexagon head bolt (2) or the hexagon nut (3).