To repair a component very easily without removing it from a vehicle body by filling a water-swelling resin composition into a scar occurring portion from the outside of an outside plate section where a scar occurs, and smoothing this filled portion.

The outside faces of an outside plate section on both sides of the portion of a scar (penetrating a raw material) occurring on a bumper fitted to the whole face of a car are ground into a V-shape, for example. The outside peripheral section is ground to a nearly uniform depth in parallel with the surface. After the surface is degreased, a water-swelling resin composition is pressed by hands to the scar occurring portion, V-shaped ground portion and the portion ground to the uniform depth while hot air is sprayed for heating, and part of the resin composition is protruded to the back side of the outside plate section through the space section of the scar occurring portion. The surface of the outside water-swelling resin composition is adjusted to a smooth face to be flush with the other face, then the resin composition is hardened to accomplish the repair.