NOVELTY - A fuel cell system comprises a fuel circulation supply unit for supplying a fuel to an anode of a direct borohydride fuel cell (DBFC) from a fuel storage unit; a polymer electrolyte membrane fuel cell (PEMFC) receiving hydrogen generated from the anode as a fuel; and a hydrogen controller for controlling hydrogen generated from the anode in correspondence to an amount required by the PEMFC and supplying hydrogen to the PEMFC.    USE - As fuel cell system.    ADVANTAGE - A fuel of the DBFC is circulation-supplied to maximize a fuel usage. Also, since hydrogen generated at the anode of the DBFC is used as a fuel source of the PEMFC, an efficiency of electric energy outputted from the system based on a fuel supplied to the system is increased and the entire system is more simplified. Additionally, since a pressure and an amount of hydrogen generated at the anode of the DBFC are controlled by the hydrogen controller in correspondence to a pressure and an amount required by the PEMFC to be supplied to the PEMFC, the system is operated more stably and effectively and the stability of the system is enhanced. Also, since the DBFC and the PEMFC are sequentially stopped at the time of stopping the entire system, hydrogen remaining in the system except the fuel tank is utilized as maximum as possible to perform a purge-driving of the system. Therefore, a fuel usage amount is minimized and a system efficiency is increased. Additionally, since an oxygen concentration and a hydrogen concentration are measured to be informed to the user while the system is operated and an excessive charge of electric energy is prevented, the user's safety is more increased.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel storage unit (110); a DBFC (200); a fuel circulation supply unit (300) for circulation-supplying a fuel to an anode of the DBFC from the fuel storage unit; a PEMFC (400) receiving hydrogen generated from the anode of the DBFC as a fuel; a hydrogen controller (500) for controlling hydrogen generated from the anode of the DBFC in correspondence to an amount required by the PEMFC and supplying the hydrogen to the PEMFC; and an air supply unit (600) for supplying air to a cathode of the DBFC and a cathode of the PEMFC. An INDEPENDENT CLAIM is also included for a method for controlling a fuel cell system comprising driving a DBFC; controlling hydrogen, a byproduct generated at an anode of the DBFC after a reaction as a preset amount; and driving a PEMFC by receiving hydrogen having an amount set in the hydrogen controlling step and thus by using the hydrogen as a fuel.    DESCRIPTION OF DRAWING(S) - The figure shows a piping diagram of a fuel cell system.    Fuel storage unit (110)    Fuel tank (110)    DBFC (200)    Fuel circulation supply unit (300)    First line (310)    Gas/liquid separator (320)    Second line (330)    Fuel pump (340)    Recycle pump (350)    Third line (371)    Three-way valve (372)    Fourth line (373)    First discharge line (374)    First purge valve (375)    PEMFC (400)    Third discharge line (411)    Fourth discharge line (431)    Humidity exchanger (432)    Evaporator (433)    Hydrogen controller (500)    Connection line (510)    Hydrogen reservoir (520)    First pressure sensor (531)    Second pressure sensor (532)    Second valve (533)    First compressor (534)    Third valve (541)    Regulator (542)    Relief valve (544)    Air supply unit (600)    Air distributor (610)    First distribution line (620)    Second distribution line (630)    Air introducing line (640)    Second compressor (650)    Air filter (660)    Humidity supply unit (670)