A purification method in which, from a specific polyoxyalkylene derivative having a molecular weight of 8,800 to 100,000, an impurity differing in the number of hydroxyl groups is separated, the method including steps (A), (B), (C), and (D). Step (A): a step in which an aprotic organic solvent is used in an amount at least 5 times by weight the amount of the polyoxyalkylene derivative to dissolve the polyoxyalkylene derivative therein and give a solution; step (B): a step in which an adsorbent comprising an oxide containing at least one of aluminum and silicon is added to the solution in an amount 0.5 to 5 times by weight the amount of the polyoxyalkylene derivative to thereby yield a slurry; step (C): a step in which the slurry is stirred at 25° C. or higher; step (D): a step in which the polyoxyalkylene derivative is recovered from the slurry.