PURPOSE: To obtain the titled composition markedly outstanding dispersibility, capable of giving tough coating-film, by chain extension, using a low molecular weight primary diol, of urethane prepolymer in the presence of low molecular weight pyroxylin and/or carboxylated pyroxylin.

CONSTITUTION: The objective composition outstanding in magnetic powder dispersibility can be obtained by chain extension of (A) a prepolymer prepared by reaction between (i) a polyol with a molecular weight 500W3,000 such as polyethylene adipate and (ii) equimolar or more amount of an organic diisocyanate compound such as 2,6-tolylene diisocyanate, in the presence of (B) a low molecular weight pyroxylin and/or carboxylated one each having an average polymerization degree ≤30, using (C) a diol having two primary OH groups which a molecular weight ≤200. This composition consists of a composite made up of the component B and the urethane.