The first agent for hair modification is appliable on hair in a reduction step for a hair modification treatment method including a reduction step； and a heating step of bringing the hair obtainable after the reduction step into contact with a heating element set at a temperature of 70° C. or higher, the first agent including a reducing agent and N-acetylglucosamine. The hair modification treatment method includes a reduction step of applying, on hair, a first agent for hair modification including a reducing agent and N-acetylglucosamine； and a heating step of bringing the hair obtainable after the reduction step into contact with a heating element set at a temperature of 70° C. or higher.
