A polyphosphazene derivative and an aromatic ring compound are bonded to one another to obtain an intermediate product. Subsequently, anhydrous sulfuric acid is added in such an amount that the molar ratio is 2 to 8 with respect to a repeating unit of the intermediate product. Further, SO.sub.3 and the intermediate product are reacted with each other to sulfonate the intermediate product. The sulfonation is performed under a condition in which the temperature range is -20 to 0.degree. C. and the total time is 5 to 30 hours. Accordingly, a sulfonated polyphosphazene derivative is obtained, in which the number average molecular weight is not less than 25,000 and the ion exchange capacity exceeds 3.0 mmol/g.