An agent for hair deforming treatment comprising the following components (A), (B) and (C) : (A) glyoxylic acid, or a hydrate or a salt thereof, (B) resorcin, and (C) water, wherein the molar ratio of the content of the component (B) to the content of the component (A), (B)/(A), is 0.2 or more and 5 or less.