Damaged hair shafts or defective hair shafts are treated by: (a) supplying moisture to the hair shafts to open the flaps in the cuticle layer; (b) coating the hair shafts with amino acids or peptides effective in supplying nutrients to the hair; and (c) inserting the hair shafts between heat plates and heating to thereby compress the hair shafts and accelerate the penetration of the amino acids or peptides into the cuticle layer, followed by closing the opening among flaps. The amino acids or peptides, which have penetrated into the hair, supply nutrients to the cortex. The hair shafts may be subjected to hair styling.