Photoelectrochemical carbon dioxide reduction was carried out at a p-type Si(111) electrode modified with a viologen molecular layer and [AuCl4](-) or [PdCl4](2-). It was proven that the reduction reaction was mediated by viologen moiety and while CO2 reduction was dominant at the Si(111) electrode modified with [PdCl4](2-) in the potential region where viologen moiety was in the first reduced state, it became dominant at the electrode modified with [AuCl4](-) when viologen moiety became the second reduced state. FT-IR measurement confirmed the formation of formic acid/formate ion at the [PdCl4](2-)/viologen-modified Si electrode.