A purification method of a polyethylene glycol compound having one carboxyl group. In the method, an aqueous solution of the compound having a pH of 8 to 11 is prepared, the aqueous solution and a mixed organic solvent are mixed at a temperature of 15° C. or more and 50° C. or less and the mixture is separated into an organic layer and an aqueous layer. In the mixed organic solvent, a ratio of solvent I is from 25 to 60% by mass, and a ratio of solvent II is from 75 to 40% by mass. Solvent I is a hydrocarbon, solvent II is a halogenated hydrocarbon, a mixing ratio Y (organic solvent I/organic solvent II), a temperature T (° C.) at mixing and a weight average molecular weight M of the compound satisfy a relationship of 3.11×10−6M+0.084≤Y−0.0135T≤−1.77×10−6M+0.404.