To provide a binder for a storage battery device whereby favorable adhesion is obtainable and swelling by an electrolytic solution can favorably be suppressed. A binder for a storage battery device, which is made of a fluorinated copolymer comprising structural units (A), structural units (B) and structural units (C), wherein the molar ratio of the structural units (C) to the total of all the structural units excluding the structural units (C) is from 0.01/100 to 3/100: structural units (A): structural units derived from a monomer selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride and chlorotrifluoroethylene; structural units (B): structural units derived from ethylene or propylene; and structural units (C): structural units derived from a C5-30 organic compound having at least two double bonds and at least one of the double bonds being a double bond of a vinyl ether group or a double bond of a vinyl ester group.