NOVELTY - A fluorosulfonyl group-containing monomer compound is new.    USE - For producing fluorosulfonyl group-containing polymer, which is useful for producing a polymer containing sulfonate groups or sulfonic acid groups, where sulfonic acid group-containing polymer is used in electrolyte material for polymer electrolyte fuel cells, for an application to a brine electrolysis, for water electrolysis, hydrogen peroxide production, ozone production and waste acid recovery, for cation exchange membrane for electrodialysis to be used for desalination or salt production, for polymer electrolyte for lithium ion cell, solid acid catalyst, cation exchange resin, sensor using modified electrodes, ion exchange filter for removing trace amount of ions in an air, and actuator, for various electrochemical processes, for membrane for diffusion dialysis to be used for separation and purification of acid, base or salt, charged porous membrane for protein separation (e.g. charged reverse osmosis membrane, charged ultrafiltration membrane and charged microfiltration membrane), dehumidifying membrane and humidifying membrane.    ADVANTAGE - The fluorosulfonyl group-containing monomer has a high polymerization reactivity for producing fluorosulfonyl group-containing polymer. The monomer is a perfluoromonomer having a perfluoro(2-methylene-1,3-dioxolane) structure having a high polymerization reactivity and two fluorosulfonyl groups, by which it is easy to obtain a copolymer having a high molecular weight by copolymerizing it with a copolymerizable monomer such as tetrafluoroethylene, and it is easy to obtain a sulfonic acid polymer having high mechanical strength and durability. Since the monomer has two fluorosulfonyl groups, it is possible to obtain a sulfonic acid polymer having a high ion-exchange capacity even if its copolymerization ratio is low, as compared with a monomer having one fluorosulfonyl group. The sulfonic acid polymer is useful as an electrolyte material for polymer electrolyte fuel cells since it has a low electric resistance owing to its high ion-exchange capacity; has a high softening point and excellent mechanical strength; and further has durability. Since the electrolyte material has a high softening point, it is possible to operate a cell at a higher temperature than conventional ones, and it is possible to make the fuel cell have a high output or improve the cooling efficiency. The electrolyte material for polymer electrolyte fuel cells, which is an electrolyte material having a high ion-exchange capacity and low resistance, and which has a higher softening point than a commonly used electrolyte material and is excellent in durability.    DETAILED DESCRIPTION - A fluorosulfonyl group-containing monomer compound of formula (III), is new.    Rf1 and Rf2=1-8C perfluoroalkylene group which optionally have an etheric oxygen atom between carbon atoms.