The Japanese electric power system has been facing to a big turning point after the outbreak of the unprecedented Great East Japan Earthquake and tsunami on March 11th 2011. New Electricity system reform is discussed by means of recent years' social requests. The hydropower generation, that is purely domestic renewable energy, plays a key role in the Japanese power system as both the peak power generation and the power system stabilizer. In addition, the pumped-storage power generation contributes to the power system with its unique characteristics. This report will introduce a key role of the hydropower generation in the Japanese power system.