A heat storage tank, which is provided in a building, on a green tract of land, in sports facilities, and in the underground portion of a parking lot for the purpose of being used as a heat supply source for an air-conditioning system. This heat storage tank consists of a plurality of cisterns, between which communicating passages composed of communicating pipes are formed. The pressure in the communicating passages is controlled through a pressure controller, such as a pressure device or a vacuum device, which thereby functions as a communication controller for controlling an operation for opening and closing the communicating passages or an operation for regulating the flow rate of the water therein.