PURPOSE: To prevent mixing of non-used accumulated heating fluid and used heating fluid and restrict a thermal loss as minimum as possible by a method wherein a partition plate moving in response to a flow rate of the thermal accumulative fluid in the thermal accumulative fluid circualtion device at the load or a heating source is arranged in the thermal accumulative tank.

CONSTITUTION: Before starting of accumulation of heat, a movable partition plate 11 is positioned at the right end in a thermal accumulative layer 1. A circulation device 7 at the thermal source sucks the non-used portion U and cools it with a refregirator, thereafter returns it as a non-used portion F to the thermal accumulative layer 1. During this operation, an output of the flow rate sensor 16 is applied to a control device 17 to provide a proper control over the driving motor 14 to cause the movable partition plate 11 to be gradually moved in a leftward direction. In this way, the non-used portion F is widened as the cold thermal accumulating fluid is increased, and when the cold accumulating fluid is to be used, the loading pump 6 in the load control circulation device 3 sucks the cold fluid 2 from the non-used portion F and sends it to the load, and thus the used portion U is expanded as the cold accumulative fluid is used.