PROBLEM TO BE SOLVED: To improve the deep dyeing property of hair and the fastness of hair dyeing in the hair dyeing treatment with an oxidative hair dye, and particularly to deeply dye the hair while maintaining the color tones exhibited by the oxidative hair dye in the hair. Provided are a hair dyeing treatment agent composition capable of improving properties and excellent storage stability over time, and a method for using the same. SOLUTION: In a hair dyeing treatment with an oxidative hair dye, a hair dye post-treatment agent composition used for hair after applying the oxidative hair dye, at least one selected from (A) aluminum salt. A post-treatment agent composition for hair dyeing, which comprises a seed and at least one selected from (B) a cationized polymer and has a pH of 2.5 or more and 4.0 or less. [Selection diagram] None【課題】酸化染毛剤による染毛処理において、毛髪の濃染性及び染毛の堅牢性を向上することができ、特に酸化染毛剤が毛髪において呈する色味を維持しつつ毛髪の濃染性を向上することができ、かつ、経時での保存安定性に優れた染毛用処理剤組成物及びその使用方法を提供する。【解決手段】酸化染毛剤による染毛処理において、酸化染毛剤を適用した後の毛髪に使用する、染毛用後処理剤組成物であって、（Ａ）アルミニウム塩から選ばれる少なくとも１種と、（Ｂ）カチオン化ポリマーから選ばれる少なくとも１種と、を含有し、ｐＨが２．５以上４．０以下である、染毛用後処理剤組成物。【選択図】なし
