The fuel cell system includes: a fuel cell 40 that receives supply of reactant gas to generate power; output characteristic updating means for updating an output characteristic of the fuel cell 40 based on output current and output voltage measured by a current sensor 140 and a voltage sensor 150; maximum power calculation means for calculating, using the output characteristic, the maximum power available at the fuel cell 40; and determination means for determining whether a value of the output characteristic is in an assumed situation where the output characteristic value is assumed to be temporarily lowered, wherein while the output characteristic value is determined by the determination means to be in the assumed situation, the maximum power calculation means calculates the maximum power using the output characteristic updated by the output characteristic updating means just before transition to the assumed situation.