PURPOSE: To enhance the economical effect in the utilization of warm water by simply and certainly preventing the mixing of liquids having different temps., by using a float enabled in the adjustment of its specific gravity in a tank.

CONSTITUTION: A float 2 is floated between the lower cold water layer 3 and the upper warm water layer 4 within a cylindrical heat accumulation tank 1 and provided so as to provide a slight gap 7 from the inner surface of the heat accumulation tank 2 so as to be capable of being floated and sunk in the heat accumulation tank 2. The specific gravity of the float 2 is adjusted by increasing and decreasing the amount of a specific gravity adjusting body 5 and the volume of an air layer 6 so as to be made smaller than that of the lower cold water layer 3 or larger than that of the upper warm water layer 4 and the boundary surface between both of them is moved and floated up and down corresponding to the specific gravities of the cold water layer 3 and the warm water layer 4 to prevent the mixing of both layers 3, 4.