A method for preparing a polyalkylene glycol derivative, comprising: - synthesizing a polyalkylene glycol derivative having the formula, by a step of reacting compound of formula (III-I) with an electrophile of formula optionally in the presence of basic compound, or by a step of reacting compound of formula (III-II) with an electrophile of formula in the presence of basic compound, and - removing water-soluble impurities in a step by passing a solution of the reaction product from step or through a column packed with a cation exchange resin and through a column packed with an anion exchange resin, or through a common column packed with a mixture of cation and anion exchange resins, passing the solution in a circulating manner to pass in said column(s) at least twice, to purify the polyalkylene glycol derivative: R 4 -R 5 -X wherein each of R 1 and R 4 independently is hydrogen, C 1 -C 20 monovalent hydrocarbon group, azido group, cyano group, protected amino group of formula (IV-I) below, protected hydroxyl group of formula (IV-II) below or acetal group of formula (IV-III) below, R 2 is C 1 -C 6 divalent hydrocarbon group, oxyalkylene group or polyoxyalkylene group, R 3 is a C 2 -C 8 divalent hydrocarbon group, n is an integer of 1 to 450, R 5 is a C 1 -C 6 divalent hydrocarbon group, M is alkali metal and X is a leaving group; wherein each of R a1 and R a2 independently is hydrogen or a protecting group for amino, at least one of R a1 and R a2 being a said amino protecting group, and provided that R a1 and R a2 may bond together to form a ring with the nitrogen atom to which they are attached, R b is a protecting group for hydroxyl, and each of R c1 and R c2 independently is C 1 -C 6 monovalent hydrocarbon group which may contain oxygen atom, and provided that R c1 and R c2 may bond together to form a cyclic acetal group, and the broken line designates a valence bond. - synthesizing a polyalkylene glycol derivative having the formula, by a step of reacting compound of formula (III-I) with an electrophile of formula optionally in the presence of basic compound, or by a step of reacting compound of formula (III-II) with an electrophile of formula in the presence of basic compound, and - removing water-soluble impurities in a step by passing a solution of the reaction product from step or through a column packed with a cation exchange resin and through a column packed with an anion exchange resin, or through a common column packed with a mixture of cation and anion exchange resins, passing the solution in a circulating manner to pass in said column(s) at least twice, to purify the polyalkylene glycol derivative: R 4 -R 5 -X wherein each of R 1 and R 4 independently is hydrogen, C 1 -C 20 monovalent hydrocarbon group, azido group, cyano group, protected amino group of formula (IV-I) below, protected hydroxyl group of formula (IV-II) below or acetal group of formula (IV-III) below, R 2 is C 1 -C 6 divalent hydrocarbon group, oxyalkylene group or polyoxyalkylene group, R 3 is a C 2 -C 8 divalent hydrocarbon group, n is an integer of 1 to 450, R 5 is a C 1 -C 6 divalent hydrocarbon group, M is alkali metal and X is a leaving group; wherein each of R a1 and R a2 independently is hydrogen or a protecting group for amino, at least one of R a1 and R a2 being a said amino protecting group, and provided that R a1 and R a2 may bond together to form a ring with the nitrogen atom to which they are attached, R b is a protecting group for hydroxyl, and each of R c1 and R c2 independently is C 1 -C 6 monovalent hydrocarbon group which may contain oxygen atom, and provided that R c1 and R c2 may bond together to form a cyclic acetal group, and the broken line designates a valence bond. each of R 1 and R 4 independently is hydrogen, C 1 -C 20 monovalent hydrocarbon group, azido group, cyano group, protected amino group of formula (IV-I) below, protected hydroxyl group of formula (IV-II) below or acetal group of formula (IV-III) below, R 2 is C 1 -C 6 divalent hydrocarbon group, oxyalkylene group or polyoxyalkylene group, R 3 is a C 2 -C 8 divalent hydrocarbon group, n is an integer of 1 to 450, R 5 is a C 1 -C 6 divalent hydrocarbon group, M is alkali metal and X is a leaving group; each of R a1 and R a2 independently is hydrogen or a protecting group for amino, at least one of R a1 and R a2 being a said amino protecting group, and provided that R a1 and R a2 may bond together to form a ring with the nitrogen atom to which they are attached, R b is a protecting group for hydroxyl, and each of R c1 and R c2 independently is C 1 -C 6 monovalent hydrocarbon group which may contain oxygen atom, and provided that R c1 and R c2 may bond together to form a cyclic acetal group, and the broken line designates a valence bond.