NOVELTY - The fuel cell has a fuel cartridge (104) storing two types of fuel differing in concentration. An edge portion of fuel injection hole is positioned at the side of fuel tank (102) of flow path for supplying the fuel to fuel tank from cartridge. A concentration sensor (101) is installed in fuel flow path, to detect the concentration of fuel in the fuel cartridge and the change of density or specific gravity of fuel. A pump (107) is controlled by a control unit (103) based on the output value of the sensor, for supplying the fuel to fuel tank from fuel cartridge.    USE - Portable fuel cell e.g. direct methanol fuel cell (DMFC) for electric vehicle, notebook type personal computer and power supply system of residence.    ADVANTAGE - The fuel residual amount detection and fuel concentration detection can be performed by using a single sensor. The electric power generation by fuel cell can be maintained steadily. The reliability of fuel cell is improved.    DETAILED DESCRIPTION - The fuel cell has a fuel cartridge storing two types of fuel differing in concentration. An edge portion of fuel injection hole is positioned at the side of fuel tank of flow path for supplying the fuel to fuel tank from cartridge. An edge portion of fuel discharge port is positioned at the side of fuel tank of fuel discharge flow path. An edge portion of fuel inlet is positioned at the side of fuel tank of flow path for supplying fuel from fuel tank to fuel cell stack. The distance between the fuel injection hole and fuel discharge port is shorter than the distance between the fuel inlet and fuel injection hole, and distance between the fuel inlet and fuel discharge port. An INDEPENDENT CLAIM is included for portable fuel cell control method involves judging the absence of fuel in fuel cell stack when the concentration value output from sensor is 100% or more, and controlling the pump so that a regular quantity of fuel is supplied to fuel tank from the fuel cartridge. The quantity of fuel in the fuel cell stack is estimated according to the oscillating period of the output value, when the output value of sensor oscillates between the concentration value of 100% or more and the concentration value less than 100%.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the portable fuel cell. (Drawing includes non-English language text)    Concentration sensor (101)    Fuel tank (102)    Control unit (103)    Fuel cartridge (104)    Pump (107)