Provided is a fuel cell system capable of making a shift of an operation state while optically controlling an output voltage and an output voltage of a fuel cell. When an ECU judges that the time when an operation should be shifted from a low-efficiency operation to a normal operation has come, the ECU performs, as preprocessing prior to a shift to a ΔV control, processing of increasing an oxidant gas supplied to a fuel cell stack by a predetermined amount. After this processing, the ECU detects output power, calculates an output power deviation, and then compares the output power deviation with a set deviation threshold. When the output power deviation exceeds the deviation threshold, the ECU carries out the ΔV control, and then carries out an I-V control. Meanwhile, when the output power deviation does not exceed the deviation threshold, the ECU judges that the time when the ΔV control is carried out has not come yet, and automatically starts the I-V control without carrying out the ΔV control.