The present invention relates to a surface-treating agent, a surface-treated powder treated by said surface-treating agent and a cosmetic containing said surface-treated powder, said surface-treating agent may endow the powder an excellent hydrophobic property and improve the washable ability.The surface-treating agent comprises a polymer containing the polymer (A) of formula (1) as constituted monomer.(wherein, R1represents hydrogen or an alkyl having 1 to 3 carbon atom (s), R2represents an alkylene having 4 to 22 carbon atom(s), X1represents-NH-group or oxygen atom, M1represents hydrogen, one-valent inorganic or organic cation.)本發明係提供在賦予粉體優越疏水性之同時可改善其沖洗性之表面處理劑、經該表面處理劑處理之表面處理粉體及含有該表面處理處理粉體之化粧料。以含有由下述通式(1)表示之單體(A)作為構成單體之聚合物所成之表面處理劑。(式中,R1表示氫原子或碳原子數1至3之烷基,R2表示碳原子數4至22之烷撐基,X1表示-NH-基或氧原子,M1表示氫原子、1價之無機或有機陽離子)。
