PROBLEM TO BE SOLVED: To provide a hair treatment agent capable of improving the permeability of a modified peptide into a hair even under pH condition of pH, and to provide a hair penetration method of the modified peptide.SOLUTION: The present invention related to the hair treatment agent, which has a pH of 3.5 to 5.5, produced by blending: (A) a modified peptide comprising one or more side chain groups selected from a side chain group having a structure represented by formula (I), a side chain group having a salt of the structure represented by formula (I), a side chain group having a structure represented by formula (II), a side chain group having a salt of the structure represented by the formula (II), a side chain group having a structure represented by the formula (III), and a side chain group having a salt of the structure represented by the formula (III) and (B) an anionic surfactant. The formula (I) is -S-S-(CH)-COOH, where n is 1 or 2, the formula (II) is -S-S-CH(CH)-COOH, and the formula (III) is -S-S-CH(COOH)-CH-COOH.SELECTED DRAWING: Figure 1
