To enhance heat efficiency by making heat exchanger area larger while reducing facility cost and reduce the difference of temperature of storage water while lowering storage water flow rate in a water tank and reduce change of temperature of the storage water to time by reducing the change of temperature of a heating part.

The water tank is equipped with a heating mechanism 1 for heating ≥10 ton storage water 2 to a prescribed temperature. The heating mechanism 1 is equipped with a warm water pipe 3 for heating the storage water 2 and a heating circulation device 4 for feeding warm water to the warm water pipe 3 and heating the warm water pipe 3. The water tank uses a concrete bottom 5 as the bottom and the warm water pipe 3 is buried in the concrete bottom 5. In the water tank, the heating circulation device 4 heats the warm water pipe 3 and the heated warm water pipe 3 heats the concrete bottom 5 and the heated concrete bottom 5 heats the storage water 2 to a prescribed temperature.