Provided is a surface treatment agent which does not use fluorine-containing monomers, particularly fluoroalkyl group-containing monomers. The surface treatment agent is a water-based emulsion which includes: a copolymer (1) which includes a first polymer formed from first monomers, and a second polymer formed from second monomers, wherein 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 do not include a halogenated olefin monomer (b), and the second monomers include the halogenated olefin monomer (b); a surfactant (2) including a nonionic surfactant; and a liquid medium (3) including water.