Provided is a surface treatment agent which does not use fluorine-containing monomers, particularly fluoroalkyl group-containing monomers. Additionally, provided is a copolymer which includes a first polymer formed from first monomers, and a second polymer formed from second monomers. The copolymer does not include fluorine. The second polymer is polymerized in the presence of the first polymer. The first monomers include a long-chain acrylate ester monomer (a) represented by the formula CH2=CA11-C(═O)—O-A12 (in the formula, A11 represents hydrogen, a monovalent organic group, or a halogen, and A12 represents a C18-30 straight-chain or branched hydrocarbon group). The first monomers and the second monomers both include a halogenated olefin monomer (b).