A fuel cell system comprising: a fuel cell stack that has a plurality of stacked fuel cells each having an anode and a cathode; a voltage sensor configured to measure a voltage of the fuel cell stack; an oxidant gas supply system configured to supply the cathode with an oxidant gas containing oxygen; a fuel gas supply system configured to supply the anode with a fuel gas; a temperature sensor configured to measure a temperature related to the fuel cell system; a secondary battery configured to be charged with power generated by the fuel cell stack and discharge the power; and a control device configured to control operation of the fuel cell system based on a measured voltage value measured by the voltage sensor, characterised in that: the control device is configured to, when the fuel cell system is started and a value measured by the temperature sensor is equal to or less than a temperature determined in advance, raise the voltage of the fuel cell stack until a voltage condition determined in advance is met, by causing the oxidant gas supply system to operate and supplying the cathode with the oxidant gas before current sweep from the fuel cell stack is started, and execute warm-up operation in which a temperature of the fuel cell stack is raised, by starting the current sweep when the measured voltage value meets the voltage condition; and the control device is configured to, when executing the warm-up operation, set a voltage command value and a current command value such that an operation point determined by a voltage value and a current value of the fuel cell stack is on an equal power line of the fuel cell stack, which indicates the same generated power as required generated power of the fuel cell stack, when the operation point is caused to transition in at least a part of a transition period from the start of the current sweep until the operation point reaches a target operation point determined by a target voltage value and a target current value during the warm-up operation. a fuel cell stack that has a plurality of stacked fuel cells each having an anode and a cathode; a voltage sensor configured to measure a voltage of the fuel cell stack; an oxidant gas supply system configured to supply the cathode with an oxidant gas containing oxygen; a fuel gas supply system configured to supply the anode with a fuel gas; a temperature sensor configured to measure a temperature related to the fuel cell system; a secondary battery configured to be charged with power generated by the fuel cell stack and discharge the power; and a control device configured to control operation of the fuel cell system based on a measured voltage value measured by the voltage sensor, characterised in that: the control device is configured to, when the fuel cell system is started and a value measured by the temperature sensor is equal to or less than a temperature determined in advance, raise the voltage of the fuel cell stack until a voltage condition determined in advance is met, by causing the oxidant gas supply system to operate and supplying the cathode with the oxidant gas before current sweep from the fuel cell stack is started, and execute warm-up operation in which a temperature of the fuel cell stack is raised, by starting the current sweep when the measured voltage value meets the voltage condition; and the control device is configured to, when executing the warm-up operation, set a voltage command value and a current command value such that an operation point determined by a voltage value and a current value of the fuel cell stack is on an equal power line of the fuel cell stack, which indicates the same generated power as required generated power of the fuel cell stack, when the operation point is caused to transition in at least a part of a transition period from the start of the current sweep until the operation point reaches a target operation point determined by a target voltage value and a target current value during the warm-up operation. the control device is configured to, when the fuel cell system is started and a value measured by the temperature sensor is equal to or less than a temperature determined in advance, raise the voltage of the fuel cell stack until a voltage condition determined in advance is met, by causing the oxidant gas supply system to operate and supplying the cathode with the oxidant gas before current sweep from the fuel cell stack is started, and execute warm-up operation in which a temperature of the fuel cell stack is raised, by starting the current sweep when the measured voltage value meets the voltage condition; and raise the voltage of the fuel cell stack until a voltage condition determined in advance is met, by causing the oxidant gas supply system to operate and supplying the cathode with the oxidant gas before current sweep from the fuel cell stack is started, and execute warm-up operation in which a temperature of the fuel cell stack is raised, by starting the current sweep when the measured voltage value meets the voltage condition; and the control device is configured to, when executing the warm-up operation, set a voltage command value and a current command value such that an operation point determined by a voltage value and a current value of the fuel cell stack is on an equal power line of the fuel cell stack, which indicates the same generated power as required generated power of the fuel cell stack, when the operation point is caused to transition in at least a part of a transition period from the start of the current sweep until the operation point reaches a target operation point determined by a target voltage value and a target current value during the warm-up operation.