NOVELTY - A fuel cell including electricity generating units each having electrolyte and on both sides of which fuel electrode and air electrode are respectively disposed, comprises a fuel concentration adjusting section (50) for adjusting concentration of fuel supplied to fuel electrode in accordance with output power density at upstream-side generating section (32) and output power density at downstream-side generating section (33).    USE - The fuel cell is used as power generating device for generating electric power by electrochemically reacting fuel with oxygen (oxidant gas). The fuel cell can be used as drive power source for driving an automobile, and a household cogeneration system. The fuel cell is used as drive power source in e.g. portable electronic device, such as notebook personal computer, portable phone and personal digital assistant.    ADVANTAGE - There is no necessity for providing a concentration sensor in each of the generating sections. Consequently, simplification of configuration of and reduction in the size of the fuel cell may be achieved. Methanol to be supplied is preliminary mixed with water, so that energy density of fuel itself is reduced.    DETAILED DESCRIPTION - A fuel cell including electricity generating units, each having an electrolyte, and on both sides of which a fuel electrode and an air electrode are respectively disposed, comprises:    (1) an upstream-side generating section formed by electricity generating units located at an upstream-side in a direction of flow of fuel;    (2) a downstream-side generating section formed by electricity generating units located at a downstream-side in the direction of flow of fuel; and    (3) a fuel concentration adjusting section for adjusting a concentration of fuel supplied to the fuel electrode in accordance with an output power density at the upstream-side generating section and an output power density at the downstream-side generating section.    An INDEPENDENT CLAIM is included for a method of operating a fuel cell, comprising adjusting a concentration of fuel to be supplied to the fuel cell in accordance with an output power density at an upstream-side electricity generating units and an output power density at a downstream-side electricity generating units, where the upstream-side electricity generating units being disposed at an upstream-side in a flow direction of the fuel, the downstream-side electricity generating units being disposed at a downstream-side in the flow direction of the fuel.    DESCRIPTION OF DRAWING(S) - The figure a block diagram illustrating the configuration of a fuel cell.    Upstream-side generating section (32)    Downstream-side generating section (33)    Fuel supply section (40)    Fuel tank (41)    Fuel concentration adjusting section (50)    Water collection section (70)