A fuel cell system including a plurality of single-cell units, each of which include an electrolyte membrane disposed between an anode and a cathode and which generate electric power by a reaction of a fuel gas in the anode and an oxidizer gas in the cathode, and a fuel cell in-which the plurality of single-cell units are stacked. A plurality of voltage measurement devices measure the voltages of the single-cell units, respectively. A control unit determines the humidification state of the fuel cell based on the voltage of a second single-cell unit wherein when the voltage of the second single-cell is equal to or lower than an insufficient-humidification-determining voltage which is higher than the predetermined voltage the control unit determines that humidification of the fuel cell is insufficient, disposed in the vicinity of a first single-cell unit, the first single-cell unit having a voltage that is equal to or lower than a predetermined voltage.