NOVELTY - Fuel cell system (100) comprises fuel cell stack (10) to generate electrical energy by reaction of fuel and oxidant; fuel supply (30) and oxidant supply (20); where fuel supply (30) having mixer (32) including inflow port (331), to mix fuel and water to generate diluted fuel and supply diluted fuel to fuel cell stack. The fuel cell system further comprises gas/liquid separator (41) for recovering water and non-reacted fuel from among mixture of gas and liquid output by fuel cell stack.    USE - As fuel cell system such as direct methanol fuel cell (DMFC), for generating electrical energy by electrochemical reaction between fuel and oxidant.    ADVANTAGE - The fuel cell system is compact and has high per-weight energy density and fuel efficiency. The fuel cell system does not have additional inflow port for supplying deionized water to mixer and fuel cartridge is replaceable with deionized water cartridge. The fuel cell system can easily perform initial drive in case of long-term dry-out without additional self-priming logic eliminating the problem of self-priming of fuel supply pump as in prior art fuel cell system. The present fuel cell system can prevent the risk of fluctuation and divergence of concentration of fuel when capacity of mixer is reduced to half compared to prior art systems since opening/closing change time of fuel inflow port and water inflow port of concentration controller is short.    DETAILED DESCRIPTION - Fuel cell system (100) comprises fuel cell stack (10) to generate electrical energy by reaction of fuel and oxidant; fuel supply (30) and oxidant supply (20) for the fuel cell stack; where fuel supply having mixer (32) including inflow port (331), to mix fuel and water to generate diluted fuel and supply diluted fuel to fuel cell stack. The fuel cell system further comprises gas/liquid separator (41) for recovering water and non-reacted fuel from among mixture of gas and liquid output by fuel cell stack; where fuel supply further comprises concentration controller (33) to control amount of fuel and water supplied to mixer; circulation pump (34) to supply fuel and water output; and concentration sensor (35) connected to concentration controller, to sense concentration of fuel in diluted fuel supplied to fuel cell stack.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of fuel cell system.    Fuel cell stack (10)    Oxidant supply (20)    Fuel supply (30)    Mixer (32)    Concentration controller (33)    Circulation pump (34)    Concentration sensor (35)    Gas/liquid separator (41)    Fuel cell system (100)    Inflow port (321)    Fuel inflow port (331)    Water inflow port (332)