PROBLEM TO BE SOLVED: To provide a treatment agent for hair deformation and a treatment method for hair deformation capable of forming waved hair and correcting curly hair by significantly reducing an amount of an oxidizing agent to be used or without substantially using an oxidizing agent.SOLUTION: There is provided a treatment agent for hair deformation which is used for hair treated by a first agent for hair deformation to deform hair without using an agent blended with 1.0 wt.% or more of an oxidizing agent and in which at least any one of dithiodiglycolic acid or a salt thereof is blended.
