A method for producing a polyoxyalkylene derivative including: step (A) in which a polyoxyalkylene derivative having hydroxyl group(s) is reacted with a phthalimide derivative in the presence of an azo reagent and a phosphine reagent; step (B) in which the phthalimide groups of the compound obtained in step (A) are deprotected using a compound having a primary amino group; and step (C) in which impurities derived from the phthalimide derivative and the deprotection reagent are removed by adsorption or extraction.