Ken’ichi Matsumoto discusses “Energy security performance in Japan.” Matsumoto shows that since Japan is poor in energy sources and because the energy situation in Japan will be tougher in the future, securing its energy supply will be a more important issue for Japan. Ken’ichi Matsumoto evaluates changes in the historical energy security performance and then analyzes energy security in the future under several scenarios for Japan. From the historical evaluation, it is shown that energy security performance evaluated by three energy security indicators improved over time, but energy security declined from 2011 due to the Fukushima nuclear disaster. This means that diversity of primary energy sources, including nuclear power, is important for high energy security performance, as measured by the three indicators introduced in this study. From the scenario analysis, energy security will improve under the future scenarios discussed in this chapter. It is suggested that energy balances mentioned above and also energy saving can improve the energy security performance of Japan compared to the historical situation.