Global warming caused by the increase of atmospheric CO2 concentration is regarded as a serious environmental issue. Due to uncertainty relating to the mechanisms of the terrestrial carbon cycle and climate system, solution of the problem is considered to be extremely complicated. In such a context, to conduct a decision analysis under uncertain CO2 concentration control policies, we have developed a global energy model which minimises the expected value of discounted total energy system costs up to the year 2055. In this work, we analyse an optimal future global energy system, particularly focusing on the Japanese electric power sector and present insight into an optimal strategy for energy system development under uncertain CO2 regulation policies.