PURPOSE: To perform a cooling operation efficiently and economically in a small space by a method wherein negative heat is stored by utilizing the latent heat of ice and the transfer of the negative heat is made by the intermediary of brine.

CONSTITUTION: A heat storage tank 10 is formed of a heat insulating wall and water is filled into the tank. Further, brine supplied from a cooler or an air conditioner is circulated through an ice machine 12 provided in the heat storage tank 10. Accordingly, the brine cooled by the cooler flows into the ice machine 12 so that ice is produced around the ice machine (heat storage). Then, when the cooling operation is performed, the brine flows into the air conditioner from the ice making machine 12 so as to exchange heat in the air conditioner to thereby cool the circumference of the air conditioner. Thus the ice heat storage unit stores heat through the ice having a heat storing power of as high as 80k cal/kg of heat of fusion and also it can freeze almost 50W100% of the water 11 in the heat storage tank 10 so that an extremely high heat storage efficiency is obtained.