To provide surface-treating agents that can provide excellent hydrophobicity to powder and can improve its rinsability, to provide surface-treated powders that are treated with the surface-treating agent, and to provide cosmetics that comprise the surface-treated powder. A surface-treating agent consisting of a polymer which comprises a monomer (A) represented by the general formula (1) described below as a constituent monomer.(wherein R1 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, R2 represents an alkylene group having 4 to 22 carbon atoms, X1 represents an —NH— group or an oxygen atom, and M1 represents a hydrogen atom or a monovalent inorganic or organic cation.)
