A purification method of separating impurities from a carboxyl group-containing polyoxyethylene derivative having a molecular weight of 2,000 to 100,000, as measured by GPC which is represented by the general formula: wherein Z is a residual group of a polyhydric alcohol having 2 to 8 active hydrogen atoms, an amino acid or a peptide; Y 1 and Y 2 are an ether bond, an amide bond, an ester bond, a urethane bond, a carbonate bond, a secondary amino group, a thioether bond, a disulfide bond, a thioester bond, or an alkylene group containing them; Polymer 1 and Polymer 2 are a linear polyoxyethylene chain or a branched polyoxyethylene chain; R is a hydrocarbon group having 1 to 7 carbon atoms or an acetal group having 3 to 9 carbon atoms; X is an amino group, a protected amino group, a carboxyl group, a protected carboxyl group, an aldehyde group, a protected aldehyde group, a hydroxyl group, a protected hydroxyl group, a thiol group, a protected thiol group, a sulfonyl group, a vinyl group, an allyl group, a cyano group, or an alkylene group containing them; m and n are each as follows: m = 1 or 0, n = 1 or 0, and 1 ≤ m+n; i and k are each as follows: 1 ≤ i ≤ 5 and k = 1 or 0; a, b, and c are each an integer satisfying the following: 0 ≤ a, 0 ≤ b, 1 ≤ c and 1 ≤ a+b, and 2 ≤ a+b+c ≤ 8; in case where a, b, and c are an integer of 2 or more, Y 1 and Y 2, Polymer 1, Polymer 2, R, X, m, n, i, and k may be same or different in the molecule, said method comprising the following steps to: a step of dissolving the polyoxyethylene derivative to form a solution using an organic solvent selected from the group consisting of toluene, xylene, benzene, ethyl acetate and butyl acetate in an amount 5 times by mass or more the amount of the polyoxyethylene derivative, a step of forming a slurry by adding to the solution an inorganic adsorbent containing at least one of an oxide and a hydroxide of one or more elements selected from the group consisting of magnesium, silicon, and aluminum in an amount 0.5 to 10 times by mass the amount of the polyoxyethylene derivative, a step of stirring the slurry at a temperature of 25°C or higher, a step of adding an organic solvent selected from the group consisting of toluene, xylene, benzene, ethyl acetate and butyl acetate to a filtration cake obtained by filtration of the slurry and further performing filtration, and a step of adding to the filtration cake an organic solvent selected from the group consisting of methanol, ethanol and 2-propanol in an amount 5 times by mass or more the amount of the polyoxyethylene derivative, performing filtration, and recovering the polyoxyethylene derivative from the resulting filtrate. a step of dissolving the polyoxyethylene derivative to form a solution using an organic solvent selected from the group consisting of toluene, xylene, benzene, ethyl acetate and butyl acetate in an amount 5 times by mass or more the amount of the polyoxyethylene derivative, a step of forming a slurry by adding to the solution an inorganic adsorbent containing at least one of an oxide and a hydroxide of one or more elements selected from the group consisting of magnesium, silicon, and aluminum in an amount 0.5 to 10 times by mass the amount of the polyoxyethylene derivative, a step of stirring the slurry at a temperature of 25°C or higher, a step of adding an organic solvent selected from the group consisting of toluene, xylene, benzene, ethyl acetate and butyl acetate to a filtration cake obtained by filtration of the slurry and further performing filtration, and a step of adding to the filtration cake an organic solvent selected from the group consisting of methanol, ethanol and 2-propanol in an amount 5 times by mass or more the amount of the polyoxyethylene derivative, performing filtration, and recovering the polyoxyethylene derivative from the resulting filtrate.