PURPOSE: To permit the storage of liquefied natural gas in a small tank by a method wherein high-pressure evaporated liquefied natural gas is given to an expansion turbine to generate cold heat and the cold heat is reserved into a cold heat accumulating means utilizing the latent heat and sensitive heat of coagulation/fusion of cold heat accumulating agent while the cold heat is utilized for the liquefying and reserving of the natural gas. CONSTITUTION: Natural gas, such as city gas or the like, is introduced from a high- pressure main line 1 into an expansion turbine E1 to reduce the pressure of the same and recover a power while the liquefied natural gas after the expansion is liquefied by cold heat accumulating agent 12 and is reserved in a tank 5. The coagulating point of the cold heat accumulating agent 12 is lower than the liquefying temperature of the natural gas and the gas from the expansion turbine is cooled and liquefied by a latent heat and a sensitive heat upon the fusion and temperature rise of the cold heat accumulating agent to reserve city gas in the state of liquid at midnight wherein the using amount of the city gas is small. In a time band from morning to noon, liquefied natural gas, which is low-temperature liquid obtained by a refrigerating machine 6, is introduced into a cold heat accumulating means 7 to coagulate the cold heat accumulating agent and accumulate cold heat as a latent heat.