Problem to be solved: to provide a hair processing base material which can be used to form a film on the surface of the hair, and this film is easy to obtain water resistance.Solution: the hair processing base comprises a copolymer of unsaturated monomer (m) containing a Class 3 cationic unsaturated monomer (a), an anionic unsaturated monomer (b), and a hydrophobic ethylenic unsaturated monomer (c).The amount of the class III unsaturated unsaturated monomer (a) is less than 3 parts by weight and less than 20 parts by weight of 100 parts by mass of unsaturated monomer (m).The amount of the anionic unsaturated monomer (b) is 1 mass or more and 5 parts by weight or less with respect to 100 mass parts of the unsaturated monomer (m).The amount of the ethylenically unsaturated monomer (c) is 35 to 60 parts by weight or less with respect to 100 mass parts of the unsaturated monomer (m).No selection【課題】毛髪の表面に皮膜を形成するために使用でき、かつこの皮膜が耐水性を得やすい、毛髪処理用基剤を提供する。【解決手段】毛髪処理用基剤は、３級カチオン性不飽和単量体（Ａ）と、アニオン性不飽和単量体（Ｂ）と、疎水性のエチレン性不飽和単量体（Ｃ）とを含む不飽和単量体（Ｍ）の共重合体を含む。３級カチオン性不飽和単量体（Ａ）の量は、不飽和単量体（Ｍ）１００質量部に対して、３質量部以上２０質量部以下である。アニオン性不飽和単量体（Ｂ）の量は、不飽和単量体（Ｍ）１００質量部に対して、１質量部以上５質量部以下である。エチレン性不飽和単量体（Ｃ）の量は、不飽和単量体（Ｍ）１００質量部に対して、３５質量部以上６０質量部以下である。【選択図】なし
