PROBLEM TO BE SOLVED: To provide a treatment agent for hair transformation which can suppress deformation of the formed hair shape.SOLUTION: An intermediate treatment step is inserted between a reduction step of applying to the hair a first agent containing a reducing agent, and an oxidation step of applying to the hair a second agent containing an oxidant agent. In this intermediate treatment step, the intermediate treatment agent containing a modified peptide comprising a structure represented by the following formula (Ia)-(Ic) and one or more kinds of a side group having a unit selected from a salt of the structures thereof is applied to the hair, and the hair concerned is heated. -S-S-(CH)-COOH (Ia) (in the formula (Ia), n is 1 or 2.) -S-S-CH(CH)-COOH (Ib) -S-S-CH(COOH)-CH-COOH (Ic)
