To provide a technology capable of suppressing pumping hydrogen from being generated at a warm-up operation of a fuel cell system.SOLUTION: A fuel cell system comprises: a fuel cell stack; and a controller that controls an operation of the fuel cell system by using a measurement voltage value measured by a voltage sensor. In a case where a measurement value of a temperature sensor is equal to or less than a predetermined temperature at initiation of the fuel cell system, the controller supplies an oxidant gas to a cathode to rise a voltage of the fuel cell stack until a predetermined voltage condition is satisfied before starting current sweeping. In a case of shifting an operation point of the fuel cell stack in at least a part of a transition period, the controller sets a voltage command value and a current command value so that the operation point is on an equal power line of the fuel cell stack.SELECTED DRAWING: Figure 6