A new photo-bioreactor system in which a photocatalysis in a light stage is separated from a biocatalysis in a dark stage is proposed for an artificial photosynthesis system with a high selectivity of products through the use of light energy without denaturation of enzyme due to light irradiation. To realize the concept, an electron transfer system applicable to the mentioned photo-bioreactor was studied. In this study, TiO2 particles and formate dehydrogenase (FDH) were used as a photocatalyst and a catalyst of the CO2 fixation, respectively. Methyl viologen, NAD+ and diaphorase (DAH) were used to mediate electron transfer from TiO2 to FDH. From PH activity profiles of the enzymes, both FDH and DAH were most active around PH 7, and coupling of these reactions was capable at that PH value. In the coupled reaction examined by using a electrode system instead of TiO2 Particles, the concentration of DAH affected the rate Of CO2 fixation and the rate had a maximum against the concentration, whilst the concentration of FDH had little effect. The proposed electron transfer system performed successfully in a batch operation. The results were so encouraging that a feasibility of the photo-bioreactor system proposed in this study was indicated.