NOVELTY - A pair of carbon monoxide (CO) reduction cells (1,2) equipped with gas diffusion electrodes with ion conductive electrolyte film (12) in between electrodes are provided. A supply path for gas passes through electrodes of cell 1 and 2. A monitor (3), monitors concentration of CO from anode outlet of cell 2. A pulse voltage is continuously applied to cell 1 and 2 by power supply (4) and (5) respectively.    USE - For electricity generation, hydrogen gas purification especially for fuel battery electricity generating system (claimed).    ADVANTAGE - CO is selectively oxidized and reduced efficiently and reliably. Consumption of hydrogen and electric power are prevented. The poisoning of catalyst in the fuel electrode due to CO is restrained hence, electricity can be generated efficiently.    DETAILED DESCRIPTION - Pair of CO reduction cells equipped with gas diffusion electrodes with ion conductive electrolyte film in between the electrodes in both cells are provided. A pulse voltage is impressed on both cells by power supply 4 and 5. A supply path for gas passes through electrodes of cell 1 to cell 2. The CO concentration monitor, monitors the concentration of CO from the anode outlet of cell 2. A pulse voltage is continuously impressed to cell 1 and voltage is applied to cell 2 on the basis of signal from CO concentration monitor and pulse voltage is impressed intermittently. CO is made to react with water in the anode of cell 1 and 2, generating carbon dioxide, proton, and electron. Hydrogen is generated by reaction of proton and electron in the cathode of cell 1 and 2. An INDEPENDENT CLAIM is also included for fuel battery electricity generation system. A heated gas is modified by a modifier and the modified heated gas is reduced in the CO concentration reducing apparatus. After reducing, the reduced gas is supplied to the fuel electrodes of fuel battery which is supplied with oxidizing gas to the oxidizing electrode, and electricity is generated.    DESCRIPTION OF DRAWING(S) - The figure shows the composition of CO concentration reduction apparatus.    Reduction cells (1,2)    Monitor (3)    Power supplies (4,5)    Ion conductive electrolyte films (12)