A fuel cell system (300) in which, at the time of starting up a first fuel cell set (109a) and a second fuel cell set (109b), one of a first air supply unit (102a) or a second air supply unit (102b) is driven and then the another air supply units is driven so that electric power of the another air supply units reaches a peak after an output voltage of the one of the fuel cell stacks reaches the first output value.