NOVELTY - A direct oxidation fuel cell system comprises: direct oxidation fuel cell having cell with anode, cathode, and polymer electrolyte membrane; fuel tank for storing liquid organic fuel or its aqueous solution; fuel supply path for supplying liquid organic fuel; anode-side discharge path for discharging fluid from anode; anode-side gas-liquid detector; anode-side switching valve downstream of anode-side gas-liquid detector; anode-side recovery path for recovering liquid contained in the fluid discharged from the anode; oxidant supply path; cathode-side discharge path; and controller.    USE - Used as power source for portable small-sized electronic appliances, such as cellular phones, personal digital assistants, notebook personal computers, and video cameras, and also as power source for electric scooters.    ADVANTAGE - The fuel cell system is small and provides high energy densities and higher user conveniences.    DETAILED DESCRIPTION - The direct oxidation fuel cell system comprises direct oxidation fuel cell having cell with anode, cathode, and polymer electrolyte membrane between anode and cathode; fuel tank for storing liquid organic fuel (methanol) or its aqueous solution; fuel supply path for supplying liquid organic fuel from fuel tank to the anode; anode-side discharge path for discharging fluid from the anode, the fluid comprising gas and liquid that comprises a by-product and a residue of the liquid organic fuel; anode-side gas-liquid detector having a tubular part provided in the anode-side discharge path for distinguishing whether the fluid passing through the tubular part is in gaseous form or liquid form; anode-side switching valve downstream of the anode-side gas-liquid detector; anode-side recovery path for recovering the liquid contained in the fluid discharged from the anode, and branching off from the anode-side switching valve and leading to the fuel tank; oxidant supply path for supplying an oxidant to the cathode; cathode-side discharge path for discharging fluid from the cathode, the fluid comprising gas and liquid that comprises a by-product and a residue of the oxidant; and controller for switching the anode-side switching valve based on an output signal from the anode-side gas liquid detector such that the liquid contained in the fluid in the anode-side discharge path is allowed to flow into the anode-side recovery path.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram showing a structure of the fuel cell system.