In the fuel cell system (1), a change in a voltage from a cell stack (5) in relation to a fuel utilization rate is acquired and deterioration of the cell stack (5) is detected by comparing a value of the voltage with a reference value. Moreover, when it is determined that the cell stack (5) has deteriorated, the rated power of the system is decreased so that the fuel cell system (1) operates according to the deterioration state of the cell stack (5).
