An object of the present invention is to provide a novel multibranched polyoxyalkylene derivative, and an intermediate for the production thereof. Specifically, the object is to provide a multibranched polyoxyalkylene derivative which can keep a high activity of a bio-related substance modified with the multibranched polyoxyalkylene derivative and which can easily produce the modified substance an intermediate thereof and a bio-related substance to which the multibranched polyoxyalkylene derivative is bonded. The novel multibranched polyoxyalkylene derivative according to the invention is a polyoxyalkylene derivative (1) having a functional group reactive with a bio-related substance and the bio-related substance according to the invention has a structure modified with the above polyoxyalkylene derivative (1) by a reaction. Furthermore, the intermediate for the production of the novel multibranched polyoxyalkylene derivative according to the invention is a polyoxyalkylene derivative (A). wherein A 1 O and A 2 O independently represent an oxyalkylene group having 2 to 4 carbon atoms, m represents 20 to 500, n represents 15 to 700, R represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms, X represents a functional group reactive with a bio-related substance, and X 1 represents a hydroxyl group which may be protected.
