To cool a turbine constituting member during daytime generation by utilizing a low cost night power without requiring a special system for cooling.

During night, air compressed by a compressor 1 is led to a heat exchanger 8, heat exchanged with hot water supplied from a hot water tank 6 by a water supply pump 7. The heat-exchanged hot water is stored in the hot water tank, while, the heat-exchanged low temperature compressed air is stored in a CAES tank 10. During daytime generation, air in the CAES tank is heat-exchanged with turbine exhaust gas in a regenerator 14 to takeout turbine combustion air. Hot water in the hot water tank is flashed into steam in a flash tank 13 to obtain turbine cooling steam for cooling a turbine constitution member.