To store heat by nighttime power by providing low-temperature side evaporator with a tank structure and carbon dioxide that is used as the low- temperature side refrigerant and is stored in the mixed state between dry ice and a liquid in the low-temperature side evaporator.

In a low-temperature side freezing cycle 3, a carbon dioxide gas 15B that is a gaseous low-temperature side refrigerant is compressed by a low-temperature side compressor 17 and is subjected to high temperature for feeding to a low-temperature side condenser 19 and heat is removed from a high-temperature side evaporator 13 for cooling and condensing. The carbon dioxide of condensed liquid passes through a low-temperature side expansion valve 21 for reducing pressure, the inside of the low-temperature side evaporator 23 is cooled, and a gaseous carbon dioxide 15A being stored inside is hardened for forming dry ice 15C, thus increasing the ratio of the dry ice 15C to the gaseous carbon dioxide 15A in the carbon dioxide being stored inside of the low-temperature side evaporator 23 and storing heat. The liquid, low-temperature carbon dioxide inside the low-temperature side evaporator 23 is fed to a heat load side by a pump 27 to store heat.