To provide a method for quantitatively synthesizing a fullerene deriva tive having a high proton (H+)-conductivity, by a simple process and in a short time.

This method for synthesizing the fullerene derivative comprises a procedure where a fullerene molecule is subjected to cyclosulfonation and another procedure where the reaction product containing the cyclosulfonated fullene is directly thrown into water and reacted with the water so as to form the fullene derivative which has H+-dissociating hydrogensulfate ester groups or the like on carbon atoms composing the fullerene molecule.