NOVELTY - A fuel cell system comprises an electricity generating unit to generate electric energy by electrochemical reaction between a fuel and oxidizer; fuel supplying unit (140) to supply the fuel to the electricity generating unit; main concentration sensor (220) to measure a concentration of the fuel; reference cell (245) comprising a reference solution and a reference concentration sensor (240); and a drive controlling unit (160) to control operation of the electricity generating unit based on the concentration values.    USE - As fuel cell including phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte membrane fuel cells and alkaline fuel cells.    ADVANTAGE - The fuel cell system is capable of ensuring an accurate concentration sensing in spite of the temperature variation inside the system. Also is capable of correcting variation in the concentration sensing value according to temperature without having a separate temperature sensor. By implementing the fuel cell system including the reference cell, a fuel cell system is capable of performing an accurate fuel concentration measurement at a low cost. Also, it is possible to control a fuel cell system using accurate concentration values obtained by properly correcting the errors of the concentration sensing values according to temperature.    DETAILED DESCRIPTION - A fuel cell system comprises an electricity generating unit configured to generate electric energy by an electrochemical reaction between a fuel and an oxidizer; a fuel supplying unit (140) fluidly connected to the electricity generating unit and configured to supply the fuel to the electricity generating unit; a main concentration sensor (220) in fluid connection with the fuel supply unit and configured to measure a concentration of the fuel; a reference cell (245) comprising a reference solution and a reference concentration sensor (240), the reference cell configured to measure a concentration of the reference solution; and a drive controlling unit (160) coupled to an output of the main concentration sensor and an output of the reference concentration sensor, the drive controlling unit configured to control operation of the electricity generating unit based on the concentration values measured in the main concentration sensor and the reference concentration sensor.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a fuel cell system.    Fuel cell stack (110)    Fuel supplying unit (140)    Fuel tank (142)    Drive controlling unit (160)    Main concentration sensor (220)    Reference concentration sensor (240)    Reference cell (245)