In situ Fourier transform infrared spectroscopic studies have been carried out on the nature of a metal complex-immobilized polyaniline (PAn)/Prussian blue-modified electrode and on the application of this electrode to the electroreduction of CO2 in an aqueous solution. On cathodic polarization, the ring structures of PAn transformed from quinoid to benzenoid states. It was confirmed that the metal complex [1,8-dihydroxynaphthalene-3,6-disulfonato iron (II)] once doped onto the conducting polymer is not undoped in cathodic processes, and the charge balance of the polymer is kept by taking an electrolyte cation in or out. The onset potential where CO2 was accumulated with the modified electrode was 0 V vs. Ag/AgCl, and the amount of CO2 on the electrode became largest at -0.2 V. The accumulation of CO2 with the electrode mediator was caused by the bonding between the mediated PAn and CO2 through a linkage of the electrophilic carbon atom of CO2 with the nitrogen atom of the benzenoid ring. The prolonged electrolysis of CO2 at potentials more negative than -0.3 V led to the generation of C-1 similar to C-3 species involving lactic and formic acids, and the scheme of the initial reduction process was disclosed.