Provided is a method for industrially producing a polyoxyethylene derivative (1) having a plurality of hydroxy groups at a terminal. The method, which is for producing a polyoxyethylene derivative represented by formula (1), comprises steps (A), (B), (C), and (D): step (A) in which a polyhydric alcohol having five or seven hydroxy groups is subjected to a hydroxy-protecting reaction to protect four or six of the hydroxy groups by forming a cyclic benzylidene acetal, thereby obtaining a compound that has a hydroxy group at the 1-position and has a protective group having a cyclic benzylidene acetal structure； step (B) in which ethylene oxide is polymerized with the compound obtained in step (A), in an amount of 11 to 3,650 mol to obtain a polyoxyethylene derivative； step (C) in which the terminal hydroxy group of the polyoxyethylene derivative obtained in step (B) is converted to a functional group reactive with a physiologically active substance； and step (D) in which the protective group of the polyoxyethylene derivative is removed. (In formula (1), L1 is a divalent linker； X is a functional group reactive with a physiologically active substance； a is 1 or 2； and n is 11-3,650.)
