Electroreduction of carbon dioxide was homogeneously carried out using cofacial dinuclear metalloporphyrin CoTMPyP-MTPPS. The reduction was catalyzed by Co(I)TMPyP which generated at - 1.8 V vs. Ag/Ag+. More excellent catalytic activity was observed in the presence of H2O in comparison with that for mononuclear one. The catalytic activity depended on the central metal of TPPS because they worked as an electron mediator.