In this analysis, we projected Japans energy demand/supply and energy-related CO2 emissions to 2050. Our analysis of various scenarios indicated that Japans CO2 emissions in 2050 could be potentially reduced by 26-58% from the current level (FY 2005) (Figure 1). These results suggest that Japan could set a CO2 emission reduction target for 2050 at between 30% and 60%. In order to reduce CO2 emissions by 60% in 2050 from the present level, Japan will have to strongly promote energy conservation at the same pace as an annual rate of 1.9% after the oil crises (to cut primary energy demand per GDP (TPES/GDP) in 2050 by 60% from 2005) and expand the share of non-fossil energy sources in total primary energy supply in 2050 to 50% (to reduce CO2 emissions per primary energy demand (CO2/TPES) in 2050 by 40% from 2005). Concerning power generation mix in 2050, nuclear power will account for 60%, solar and other renewable energy sources for 20%, hydro power for 10% and fossil-fired generation for 10%, indicating substantial shift away from fossil fuel in electric power supply. Among the mitigation measures in the case of reducing CO2 emissions by 60% in 2050, energy conservation will make the greatest contribution to the emission reduction, being followed by solar power, nuclear power and other renewable energy sources (Figure 2). In order to realize this massive CO2 abatement, however, Japan will have to overcome technological and economic challenges including the large-scale deployment of nuclear power and renewable technologies.