PURPOSE: To obtain hot water boiling always at a high temperature by providing the inner peripheral surface of a liquid storing chamber with a pressure sensor working when a liquid storing chamber internal-section comes to set pressure with vapor generated at the time of the boiling of contained liquid, and controlling the electrification of a heater.

CONSTITUTION: On the inner peripheral surface of a vapor chamber 3a, the pressure sensor 23 of a diaphragm type is set. Directly before coming to a boiling state, much vapor S is generated in the vapor chamber 3a, and internal pressure is heightened, and when it reaches the working setting pressure of the pressure sensor 23 set in the vapor chamber 3a, then the pressure is detected, and the electrification of a water boiling heater 5a is controlled to be turned OFF. After that, the temperature of contained liquid W is retained only by heating a temperature retaining heater 5b. When the internal pressure is made lower than the working setting pressure of the pressure sensor 23 along with the slight lowering of the temperature of the contained liquid W, then the lowering of the pressure is detected by the pressure sensor 23, and the water boiling heater 5a is electrified, and the contained liquid W is heated again to be boiled.