NOVELTY - A method of driving a fuel cell device (10) involves calculating a fuel shortage based on difference between the fuel concentration detected by the concentration sensor element and predetermined desired fuel concentration; obtaining an efficiency of electricity generation of the electromotive section; calculating a fuel consumption by the electromotive section for electricity generation based on the output of the electromotive section and the obtained electricity generation efficiency; and replenishing by the supply section the mixing tank with fuel.    USE - For driving a fuel cell device (claimed) for supplying a current to an electronic device.    ADVANTAGE - The efficiency of electricity generation is obtained based on the cell stack temperature and the load current, the estimated amount of methanol consumed by the electricity generation is calculated based on the obtained electricity generation efficiency and the stack output, and the methanol supply is corrected. By doing this, an optimum amount of methanol can be supplied for replenishment. Thus, the fuel consumption can be accurately calculated based on the operating conditions of the fuel cell device, fluctuations in the ambient environment, variations in the energy consumption of the electronic device, etc. By supplying an amount of methanol equivalent to the consumption for replenishment, the concentration of the aqueous methanol solution can be stabilized at an optimum value such that the electricity generation operation of the fuel cell device is stable. Thus, using the method an optimum amount of fuel is replenished to set an appropriate fuel concentration, where stable operations and enhanced output energy can be ensured.    DETAILED DESCRIPTION - A method of driving a fuel cell device (10) which is provided with an electromotive section including anode and cathode, and generating electricity based on a chemical reaction of a fuel supplied to the anode and air supplied to the cathode, a control section which delivers an output from the electromotive section to a connected electronic device (17) based on a load current of the electronic device, a mixing tank configured to contain the fuel supplied to the anode, a fuel channel through which the fuel supplied from the mixing tank is run via the anode side of the electromotive section, an air channel through which air is run via the cathode, a replenishment container which contains a fuel with which the mixing tank is replenished, a fuel supply section which supplies the fuel in the replenishment container to the mixing tank, a concentration sensor element which is disposed in the fuel channel between the mixing tank and the electromotive section and detects a fuel concentration of an aqueous fuel solution flowing through the fuel channel, and a temperature sensor element which detects a temperature of the electromotive section, involves calculating a fuel shortage based on a difference between the fuel concentration detected by the concentration sensor element and a predetermined desired fuel concentration; obtaining an efficiency of electricity generation of the electromotive section based on the temperature of the electromotive section detected by the temperature sensor element and the load current measured by the control section; calculating a fuel consumption by the electromotive section for electricity generation based on the output of the electromotive section and the obtained electricity generation efficiency; and replenishing by the supply section the mixing tank with an amount of fuel equivalent to the sum of the calculated fuel shortage and the calculated fuel consumption, thus controlling the concentration of the fuel supplied to the electromotive section.    DESCRIPTION OF DRAWING(S) - The figure shows schematic diagram of a fuel cell device.    Fuel cell device (10)    Cell stack (12)    Fuel tank (14)    Cell control section (16)    Electronic device (17)    Circulation system (20)