This study assesses the regional energy mix potentials of Japan for maximised renewable electricity generation and reduced CO2 emission intensity in the electricity sector, in view of the nationally determined contribution (NDC) mitigation target, and the 2 degrees C rise limit target. Beginning with the deregulation of the retail electricity market, discussions have been initiated about Japanese electricity system reforms towards 2020. This paper examines the potential energy mix up to 2030 at the regional level, and identifies the need to accelerate electricity system reforms to expand the transregional access to renewable electricity generation. By analysing available data, we assess how regional renewable energy potentials can be put to effective use, and identify how electricity reform should proceed, to both capitalise on renewables and reduce carbon intensity. Finally, we report the large renewable potentials in Japan. However, in order to maximise the use of these potentials, a combination of technologies and policies are required to promote flexible grid operation, and strengthen transmission capacity and renewable priority dispatch order, as well as to introduce technology for stabilizing electricity systems supplied by renewable electricity, such as pumped storage hydropower, storage cells, and demand-response, which can store surplus energy until it is needed.