To provide an emulsifier which maintains emulsifying capability even if an anion component is decreased and which is excellent in water resistance in a dried coating film when the emulsifier is used, and to provide a resin dispersion body made using the same.

The emulsifier includes a copolymer having: a structure unit in which there are provided an ethylene oxide group and butylene oxide group; a repeating unit number m of the ethylene oxide group is 10 to 65; the repeating unit number n of the butylene oxide group is ≥1 and ≤8; and which is originated in a vinyl monopolymer (A) with radical polymerization double bonds; and a structure unit originated in a vinyl monopolymer (B) expressed by formula (1): CH₂=C(R¹)-(CO)-N(R²)-X-OH (wherein, R¹ is an alkyl group having a hydrogen atom or 1-4 carbons; R² is an alkyl group having a hydrogen atom or 1-4 carbons; and X is an alkylene group having 1-4 carbons).