Provided is a hair treatment agent having a strong hair straightening effect while also having, compared to conventional agents, an optimum pH close to those of the hair and the scalp, less likelihood of causing excessively fine hair, less likelihood of irritating the scalp, and an excellent feel when used. This hair treatment agent contains (a) glyoxylic acid and (b) gluconolactone and/or erucalactone, has a compounding ratio (component [a]:component [b], on a mass basis) of 1:1 to 20:1, and has a pH within the range of 2.0 to 3.8. It is more preferable that the compounding ratio (component [a]:component [b], on a mass basis) of the hair treatment agent be 2.5:1 to 10:1. Also, it is more preferable that the pH of the hair treatment agent be within the range of 2.5 to 3.8.本發明提供一種具有較高之毛髪矯正效果並且具有較先前更接近毛髪或頭皮之最佳pH值，且不易引起毛髪過度之收斂，對頭皮之刺激性較低且使用觸感優異之毛髪處理劑。本發明之毛髪處理劑含有(a)乙醛酸、及(b)選自葡萄糖酸內酯及芥酸內酯之1種以上，且(a)成分：(b)成分之調配比率(質量比)為1：1～20：1，pH值為2.0～3.8之範圍。毛髪處理劑中之(a)成分：(b)成分之調配比率(質量比)進而較佳為2.5：1～10：1。又，毛髪處理劑之pH值進而較佳為2.5～3.8之範圍。
