Sodium-Sulfur battery (NAS battery), which has more than 3 times of energy density compared with the conventional lead-acid battery and can be compactly established, has a great installation effects as a distributed energy storage system in the urban area which consumes big electric power. For the power company, NAS battery contributes to the load leveling, the supply capability up at the peak period, the efficient operation of the electric power equipment and the reduction of the capital expenditure. And for the customer, it is possible to enjoy the reduction of the electricity charges by utilizing nighttime electric power and the securing of a security. The contribution to the highly sophisticated information society where the higher electric power quality is desired, mainly office buildings and factories by the progress of IT, is very big. Tokyo Electric Power Company (TEPCO) developed the elementary technology of NAS battery from 1984 and ended the development of practical battery which has long-term durability and the safety and the performance verification of the megawatt scale. Finally TEPCO accomplished the practical application and commercialization of the stationary energy storage technology by NAS battery. In this paper, we introduces about conquered problems until practical application and commercialization.