NOVELTY - A fuel cell system comprises a load sensing unit for sensing a load connected to a direct borohydride fuel cell (DBFC), a polymer electrolyte membrane fuel cell (PEMFC) and a supplementary power; and a control unit for controlling a power of the DBFC, the PEMFC, and the supplementary power according to a load sensed by the load sensing unit, and selectively supplying to the load.    USE - As fuel cell system.    ADVANTAGE - Hydrogen generated at the DBFC is recycled to restrain a discharge of the explosive hydrogen and the supplementary power is selectively driven by detecting a load amount. The invention makes it possible to correspond to an instantaneous variation of a load and to stably operate the system. Also, it is possible to correspond to a drastic load variation by using the supplementary power even under a state that a capacity of the DBFC and the PEMFC is less. A cost is reduced and a compact system can be constructed. Additionally, a drastically decreased voltage of the unit cell due to a drastic load variation can be complemented by using the supplementary power, thereby gradually increasing a load amount.    DETAILED DESCRIPTION - A fuel cell system comprises a direct borohydride fuel cell for generating a power by receiving a fuel; a polymer electrolyte membrane fuel cell for generating a power by receiving hydrogen, a byproduct generated at an anode of the DBFC after a reaction, as a fuel; a supplementary power partially charged by a power generated at the DBFC and the PEMFC and discharging the charged power; a load sensing unit for sensing a load connected to the DBFC, the PEMFC, and the supplementary power; and a control unit (400) for controlling a power of the DBFC, the PEMFC, and the supplementary power according to a load sensed by the load sensing unit, and selectively supplying to the load. An INDEPENDENT CLAIM is also included for controlling a fuel cell system comprising a first step of generating a power by driving a DBFC using a power of a supplementary power at the time of an initial system driving, and supplying hydrogen, a byproduct after a reaction to a PEMFC; a second step of generating a power by driving the PEMFC using hydrogen supplied in the first step; a third step of measuring a consumption power of a load that consumes a power generated at the DBFC and the PEMFC; and a fourth step of discharging the supplementary power when the consumption power of the load measured in the third step is more than a sum of the power generated at the DBFC and the PEMFC, and charging the supplementary power when the measured consumption power of the load is less than the sum of the power generated at the DBFC and the PEMFC.    DESCRIPTION OF DRAWING(S) - The figure shows a view of a fuel cell system.    Control unit (400)