Fuel cell system and method of operating fuel cell system

A method of operating a fuel cell system comprising:

• a polymer electrolyte membrane;
• a cathode provided at one side of the polymer electrolyte membrane;
• an anode provided at the other side of the polymer electrolyte membrane;
• an oxygen supply unit configured to supply oxygen to the cathode;
• a fuel supply unit configured to supply a fuel containing methanol and water to the anode;
• a methanol supply unit configured to supply methanol to the fuel;
• a sensor which measures a methanol concentration of the fuel;
• a current measuring unit configured to measure a current flowing between the cathode and the anode;
• and
• a concentration adjusting unit configured to adjust the methanol concentration of the fuel by controlling the methanol supply unit by using an output value of the sensor, the method comprising:
• stopping both the methanol supply unit and the concentration adjusting unit;
• and
• correcting a data of a relation between the methanol concentration of the fuel and the output value of the sensor, using an output of the current measuring unit.