A fuel cell system according to the invention comprises a fuel cell stack, a fuel feeder that supplies the fuel cell stack with fuel, a rechargeable battery that is an electric storage device, a bidirectional DC/DC converter that selectively performs an operation of converting the output voltage of the rechargeable battery into a predetermined voltage and then outputting it (a discharge operation), or an operation of charging the rechargeable battery using the electric power outputted from the fuel cell stack (a charge operation), and a mode control circuit. The mode control circuit detects the output voltage of the fuel cell stack so that, when it is higher than the set value, the mode control circuit makes the bidirectional DC/DC converter perform the charge operation; when it is not higher than the set value, the mode control circuit makes the bidirectional DC/DC converter perform the discharge operation.