A fuel cell system comprising: a power generation means that includes a plurality of fuel cells (11A to 11E) that are connectable to one another in series and/or in parallel through connecting terminals (18A to 18E, 19A to 19E), a fuel gas supply conduit through which fuel electrodes (12A to 12E) of all or part of the fuel cells (11A to 11E) are connected in series and an oxidant gas supply conduit through which air electrodes (14A to 14E) of all or part of the fuel cells (11A to 11 E) are connected in series; a switching means that switches an electric connection condition between the connecting terminals (18A to 18E, 19A to 19E) and connecting means of an external load device; a fuel gas supply means that supplies the fuel gas supply conduit with a fuel gas and an oxidant gas supply means that supplies the oxidant gas supply conduit with an oxidant gas; and a load detecting means that detects a load of the external load device; characterized by further comprising a control means that is adapted to select, in accordance with an input from the load detecting means and based on an operation map indicating a relation among the number of mutually connected fuel cells (11A to 11E), the fuel cell voltage and an amount of heat radiation from all of the fuel cells (11A to 11E), a new operation point which lies in an operation range in which the amount of heat radiation from the fuel cells (11A to 11E) becomes higher than a level that keeps the heat balance of the fuel cell system, wherein the control means is further adapted to realize the number of mutually connected fuel cells (11A to 11E) and the fuel cell voltage at the new operation point selected by controlling the switching means, the fuel gas supply means and the oxidant gas supply means. a power generation means that includes a plurality of fuel cells (11A to 11E) that are connectable to one another in series and/or in parallel through connecting terminals (18A to 18E, 19A to 19E), a fuel gas supply conduit through which fuel electrodes (12A to 12E) of all or part of the fuel cells (11A to 11E) are connected in series and an oxidant gas supply conduit through which air electrodes (14A to 14E) of all or part of the fuel cells (11A to 11 E) are connected in series; a switching means that switches an electric connection condition between the connecting terminals (18A to 18E, 19A to 19E) and connecting means of an external load device; a fuel gas supply means that supplies the fuel gas supply conduit with a fuel gas and an oxidant gas supply means that supplies the oxidant gas supply conduit with an oxidant gas; and a load detecting means that detects a load of the external load device; characterized by further comprising a control means that is adapted to select, in accordance with an input from the load detecting means and based on an operation map indicating a relation among the number of mutually connected fuel cells (11A to 11E), the fuel cell voltage and an amount of heat radiation from all of the fuel cells (11A to 11E), a new operation point which lies in an operation range in which the amount of heat radiation from the fuel cells (11A to 11E) becomes higher than a level that keeps the heat balance of the fuel cell system, wherein the control means is further adapted to realize the number of mutually connected fuel cells (11A to 11E) and the fuel cell voltage at the new operation point selected by controlling the switching means, the fuel gas supply means and the oxidant gas supply means.