NOVELTY - Controlling fuel generating reaction of fuel cell (130), comprises: (a) providing first reactant; (b) activating first reactant to generate fuel; (c) adding second reactant to determine monitoring time when characteristic value of cell reaches first reference value; (d) detecting characteristic value to acquire first characteristic value; (e) proceeding to step (d) if the first value is lower than first reference value; (f) detecting characteristic value to acquire second characteristic value; and (g) proceeding to step (d) if the second value is lower than first reference value.    USE - The method is useful for controlling a fuel generating reaction of a fuel cell.    ADVANTAGE - The quantity of the second reactant to be added within a unit time may be controlled effectively and the supply of fuel may also be stably controlled. The fuel cell system does not require the use of the flow control valve, thus the manufacturing cost of the fuel cell system is reduced. The method: can be operated under different environmental temperatures so that suitable characteristic time (that is, the monitoring time) is automatically defined along with different environmental temperatures; and changes the detecting time along with the performance of the fuel system as the performance of the fuel cell system reduces, so that false determination does not occur as the performance reduces.    DETAILED DESCRIPTION - Controlling a fuel generating reaction of a fuel cell (130), comprises: (a) providing a first reactant; (b) activating the first reactant for generating a fuel to the fuel cell; (c) adding a second reactant to the first reactant to determine a monitoring time when a characteristic value of the fuel cell reaches a first reference value during activation, where the monitoring time is a time of the characteristic value changing from a second reference value to the first reference value after adding the second reactant; (d) detecting the characteristic value of the fuel cell to acquire a first characteristic value after adding the second reactant to the first reactant and after the monitoring time; (e) proceeding to the step (d) if the first characteristic value is lower than the first reference value; (f) detecting the characteristic value of the fuel cell to acquire a second characteristic value after a delay time if the first characteristic value is higher than the first reference value; and (g) proceeding to the step (d) if the second characteristic value is lower than the first reference value. INDEPENDENT CLAIMS are also included for:    (1) a computer capable of controlling the fuel generating reaction, completing the method after the computer loads and executes a computer program; and    (2) a fuel cell system (100) comprising: a chamber (110), having the first reactant; a supplying device (120), capable of determining a quantity of a second reactant supplied to the chamber according to a control signal, where the first reactant and the second reactant cause a fuel generating reaction in the chamber to generate a fuel; a fuel cell, coupled to the chamber for receiving the fuel so as to generate power; and a control unit (140), electrically connected to the supplying device and the fuel cell for providing the control signal to the supplying device and monitoring a characteristic value of the fuel cell, where the control unit performs the method of controlling the fuel generating reaction.    DESCRIPTION OF DRAWING(S) - The figure shows a frame diagram of a fuel cell system.    Fuel cell system (100)    Chamber (110)    Supplying device (120)    Fuel cell (130)    Control unit (140)