To provide a small-sized fuel cell system capable of providing a mode having a simple constitution preventing power generation of a fuel cell in an overload state, preventing a deterioration in the fuel cell system and providing a stable output from the fuel cell in the fuel cell system constituting a power source by the fuel cell. Overload states of all of single cells constituting a fuel cell are detected and an output of the fuel cell is controlled such that when the overload state is detected, the output of the fuel cell is cut or a cell in the overload state is recovered to a normal power generating state.