Nine years have passed since the Fukushima Daiichi nuclear disaster (FDND). The Japanese government has been facing the issue of striking a balance among economy, environment, and social opinions for its energy transition policy. Increasing usages of fossil fuel, natural gas, and coal can fix the energy gap left out by reduced nuclear use and stabilise Japan’s energy supply, ensuring economic growth; however, the measure would increase the global warming potential. This study applies the Fossil fuel supply security index (FFSSI) to quantify the present energy supply security in Japan and presents future scenarios of greenhouse gas emissions (GHGs) based on analysed results from the Linear Regression, Polynomial Regression, and Holt-Winters forecasting models. The driving forces of GHGs are analysed by Kaya identity to show the outlook in Japan. The aim of this study is to present the feasibility of reaching the Japanese government launched “Long-Term Energy Supply and Demand Outlook” for fiscal 2030, under Japan’s current energy supply system for policymaker’s consideration. Compared with other Asian-pacific countries (China, South Korea, Taiwan, etc.), the lacking self- sufficiency energy is the major weakness for Japan’s present energy supply system. Moreover, extrapolations based on several forecasting models indicate that the carbon dioxide emission is expected to increase in the next decade if keep continuing the present structure of the energy supply system.