Disclosed is an emulsion formed using, as an emulsifier, 0.01 to 30 parts by weight of a perfluoroalkylethyl phosphonic acid salt represented by the general formula: CnF2n+1CH2CH2P(O)(OM1)(OM2) [III] (M1: a hydrogen atom, ammonium salt, or organic amine salt, M2: an ammonium salt or organic amine salt, n: 1 to 6), based on 100 parts by weight of a perfluoropolyether oil represented by the general formula: RfO(C3F6O)p(C2F4O)q(CF2O)rRf′ [I] (Rf and Rf′: C1-C5 perfluoroalkyl groups, p+q+r: 2 to 200) or the general formula: F(CF2CF2CF2O)nCF2CF3 [II] (n: 2 to 100), or a perfluorocarbon compound. In spite of using, as an emulsifier, a compound having a perfluoroalkyl group containing 6 or less carbon atoms, which is said to have low bioaccumulation potential, the emulsion exhibits excellent emulsification stability, and therefore can be effectively used as a surface-treating agent, such as a mold-releasing agent.