[Co(bpy)(3)](3+/2+) and [Co(phen)(3)](3+/2+) redox couples were revealed to play as electron mediators for Z-scheme photocatalyst systems composed of Ru/SrTiO3:Rh and BiVO4 powders for overall water splitting under visible light irradiation. These electron mediators were effective for only the combination of SrTiO3:Rh with BiVO4. They did not work when nondoped SrTiO3 and TiO2 of H-2-evolving photocatalysts and WO3 of O-2-evolving photocatalysts were employed. These results indicated that the affinity between photocatalysts and the Co-complex electron mediators was important. The photocatalytic activity depended on pH. Neutral pH conditions gave the highest activity for overall water splitting. Overall water splitting by the present system steadily proceeded for a long time. The Z-scheme photocatalyst system was also confirmed to split water under sunlight irradiation at the rates depending on weather. Moreover, overall water splitting by the Z-scheme photocatalyst system with the Co-complex electron mediator using a reaction cell in which the Ru/SrTiO3:Rh suspension was divided from BiVO4 suspension by a membrane filter resulted in H-2 evolution separated from that of O-2.