NOVELTY - Generating power with fuel cell system including a stack (1) involves detecting voltage produced by stack; supplying liquid fuel to the stack; supplying air to the stack at first rate during start-up operation; and supplying air to the stack at second rate during rated power generation operation, where the power generation operation is performed after the start-up operation has been performed for a given time, where the second rate is set as to be required in the power generation operation and the first rate is larger than second rate.    USE - For generating power with a fuel cell system including a stack (claimed). The fuel cell such as direct methanol fuel cell (DMFC) is used as power source for mobile electrical apparatus such as small portable apparatus and personal computer.    ADVANTAGE - The fuel cell system supplies liquid fuel and air to a stack to thus cause reaction, thereby generating rated power. The fuel cell uses liquid fuel in view of suppressing degradation in performance of a stack which may otherwise occur when power generation is carried out for a long period of time. The method suppresses degradation in performance which may otherwise be caused by flooding of a cathode with water. When a fuel cell starts power generation from room temperature, cell temperature increases with the course of power generating time. As a result, diffusion of fuel and catalytic activity increase, and cell resistance decreases, thereby increasing power generation output. In the method, air is supplied upon start-up of power generation at an airflow rate higher than that of air to be supplied during rated power generation, so as to eliminate flooding of a cathode with water which causes a decrease in power output.    DETAILED DESCRIPTION - Generating a power with a fuel cell system including a stack (1) involves detecting a voltage produced by the stack; supplying a liquid fuel to the stack; supplying air to the stack at a first rate during a start-up operation; and supplying air to the stack at a second rate during a rated power generation operation, where the rated power generation operation is performed after the start-up operation has been performed for a given time, where the second rate is set as to be required in the rated power generation operation and the first rate is larger than the second rate. The step of detecting of the voltage includes directly detecting an open circuit voltage produced by the stack. The method further involves reversing air supply direction between the start-up operation and the rated power generation operation. The given time is 5-60 (preferably 5-30, especially 5-15) minutes.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of fuel cell system.    Stack (1)    Fuel circulation section (2)    Air-supply pump (3)    Gas-liquid separation tank (4)    Auxiliary air-supply pump (5)    Auxiliary power source (6)    Piping supplying methanol aqueous solution from fuel circulation section to stack (7)    Piping (8)    Piping for supplying air (9)    Piping for supplying air containing water vapor (10)