A fuel cell system comprising a cell that is capable of generating electric power and includes: a fuel electrode to which hydrogen obtained by reforming fuel gas can be supplied; an air electrode to which oxygen in air can be supplied; and an electrolyte interposed between the fuel electrode and the air electrode forallowing oxygen ions generated from oxygen in the air supplied to the air electrode to pass through to the fuel electrode, wherein the fuel cell system further comprises: a water vapor retaining means that is disposed in a flow path of the fuel gas to be supplied to the fuel electrode and which is capable of retaining water vapor generated in the fuel electrode during electric power generation by the cell, and which is capable of mixing the water vapor with the fuel gas; a fuel electrode-side terminal (2, 9) electrically connected to the fuel electrode and having formed therein a fuel channel (2a, 9a) through which the fuel flows; an air electrode-side terminal (3, 10) electrically connected to the air electrode and having formed therein an air channel (3a, 10a) through which the air flows; a fuel electrode-side current collector including an electrically conductive member interposed between the fuel electrode of the cell and both the fuel electrode-side terminal (2, 9) and the water vapor retaining means; wherein the water vapor retaining means includes a reforming catalyst for allowing a reaction of the fuel gas to proceed to thereby generate hydrogen and for allowing the hydrogen generated by the catalytic reaction using the reforming catalyst to be supplied to the fuel electrode and wherein the water vapor retaining means includes a sheet-shaped member having air permeability and flexibility and formed from a paper-like member including inorganic or organic fibres and supports the reforming catalyst. a fuel electrode to which hydrogen obtained by reforming fuel gas can be supplied; an air electrode to which oxygen in air can be supplied; and an electrolyte interposed between the fuel electrode and the air electrode forallowing oxygen ions generated from oxygen in the air supplied to the air electrode to pass through to the fuel electrode, wherein the fuel cell system further comprises: a water vapor retaining means that is disposed in a flow path of the fuel gas to be supplied to the fuel electrode and which is capable of retaining water vapor generated in the fuel electrode during electric power generation by the cell, and which is capable of mixing the water vapor with the fuel gas; a fuel electrode-side terminal (2, 9) electrically connected to the fuel electrode and having formed therein a fuel channel (2a, 9a) through which the fuel flows; an air electrode-side terminal (3, 10) electrically connected to the air electrode and having formed therein an air channel (3a, 10a) through which the air flows; a fuel electrode-side current collector including an electrically conductive member interposed between the fuel electrode of the cell and both the fuel electrode-side terminal (2, 9) and the water vapor retaining means; wherein the water vapor retaining means includes a reforming catalyst for allowing a reaction of the fuel gas to proceed to thereby generate hydrogen and for allowing the hydrogen generated by the catalytic reaction using the reforming catalyst to be supplied to the fuel electrode and wherein the water vapor retaining means includes a sheet-shaped member having air permeability and flexibility and formed from a paper-like member including inorganic or organic fibres and supports the reforming catalyst. a water vapor retaining means that is disposed in a flow path of the fuel gas to be supplied to the fuel electrode and which is capable of retaining water vapor generated in the fuel electrode during electric power generation by the cell, and which is capable of mixing the water vapor with the fuel gas; a fuel electrode-side terminal (2, 9) electrically connected to the fuel electrode and having formed therein a fuel channel (2a, 9a) through which the fuel flows; an air electrode-side terminal (3, 10) electrically connected to the air electrode and having formed therein an air channel (3a, 10a) through which the air flows; a fuel electrode-side current collector including an electrically conductive member interposed between the fuel electrode of the cell and both the fuel electrode-side terminal (2, 9) and the water vapor retaining means; wherein the water vapor retaining means includes a reforming catalyst for allowing a reaction of the fuel gas to proceed to thereby generate hydrogen and for allowing the hydrogen generated by the catalytic reaction using the reforming catalyst to be supplied to the fuel electrode and wherein the water vapor retaining means includes a sheet-shaped member having air permeability and flexibility and formed from a paper-like member including inorganic or organic fibres and supports the reforming catalyst.