This paper assesses the potential for improving the CO2 intensity of Japan’s electricity sector—the units of carbon dioxide emitted per units of electricity generated—by examining the potential for the feasibility of restarting nuclear power plants and increasing renewable electricity generation by 2030. The analysis shows that, utilizing these two strategies, it is possible to achieve the intensity that can be calculated from the emissions intensity of fossil-fuel power technology and the energy mix of Japan’s Intended Nationally Determined Contribution (INDC), and to exceed the voluntary emissions-intensity target announced by Japan’s electricity companies, under the current level of investment in electricity grid facilities. The paper focuses only on the energy mix in the generation projected for 2030, using total electricity generation estimates in the INDC and electricity demand estimates from the Long-term Energy Supply and Demand Outlook.