Formic acid formed on a SnO2 disk electrode as a reduction product of carbon dioxide was detected at a Pt ring electrode, and the fixation mechanism of carbon dioxide was investigated It was found that organic products such as HCOOH could be detected electrochemically by a rotating ring-disk electrode technique which does not require a longterm electreoreduction and offers a much more sensitive method than gas- and liquid-chromatography do. It was clarified that CO2 was reduced to HCOOH on the SnO2 surface in competition with hydrogen formation : both HCOOH and H-2 were formed quite slowly in a few seconds. The rotational speed dependence of the reaction current revealed that a considerably slow electron transfer results in the formation of some intermediates, from which HCOOH will be formed under the mass-transfer-limited condition of proton. The formation of HCOOH was found-to be most pronounced at pH 3.5.