N,N-Dimethylacetamide (DMA) was used for the first time as the reaction solvent in the photocatalytic reduction of CO2. DMA is highly stable against hydrolysis and does not produce formate even if it is hydrolyzed. We report the catalytic activities of [Ru(bpy)2(CO)2](PF6)2 (bpy = 2,2'-bipyridine) in the presence of [Ru(bpy)3](PF6)2 as a photosensitizer and 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor in DMA/water. In the photochemical CO2 reduction, carbon monoxide (CO) and formate are catalytically produced, while dihydrogen (H2) from the reduction of water is scarcely evolved. We verified that BNAH is oxidized to afford BNA dimers during the photocatalyses in DMA/water. The plots of the production for the CO2 reduction versus the water content in DMA/water show that the 10 vol % water content gives the highest amount of the reduction products, whose reaction quantum yields (Φ') are determined to be 11.6% and 3.2% for CO and formate, respectively. The results are compared with those in the N,N-dimethylformamide (DMF)/water system, which has been typically used as the solvent system for the CO2 reduction.