PURPOSE: To perform an optional setting of a boiling-up temperature and an amount of boiling-up hot water and realize them.

CONSTITUTION: Water is taken from a top part of a hot water storing tank 10 and a circulation passage 20 is disposed at an upper part of the hot water storing tank 10 and opened downwardly through a nozzle 23. The circulation passage 20 is provided with a circulation pump 21 and a heating source 22. High temperature hot water from the nozzle 23 has a reached height (h) determined by a flow speed (v) and water in a high temperature region A above an interface region K is uniformly increased, so that if a circulating amount Q is controlled, the flow speed (v) and the reached depth (h) are controlled to enable an arbitrary boiled-up amount of hot water to be attained and the boiling-up temperature at that time can be controlled according to the boiling-up time.