PURPOSE: To get a measured value which is approximate to a true value under a minimum number of times by a method wherein a water level measurement is repeated under a designated condition and if an absolute value of a subtracted value between a previous measured value and a latter measured value is within a specified range, the latter measured value is defined as a water level measured value and if the value is out of the designated range twice, the water level measurement is unabled to perform.

CONSTITUTION: A water supplying valve 22 is opened and water is supplied through a conduit 24. After a valve body 18b of a changing-over valve 18 is moved and opened, a circulation pump 16 is turned on and rotated to make a sufficient discharge of mixed air. After a designated period, the water supplying is stopped and at the same time the circulation pump 16 is turned off and stopped. A designated period is kept, a full water level 31 is measured under a still condition of the hot water surface to get an initial measured value H1. If an absolute value of differences H1 to H2 between a measured value H1 to full water level and the second time full water level measured value H2 is more than a designated value Hd, the circulation pump 16 is turned on and rotated again for a designated period of time, and after the pump is turned off, it is kept for a designated period of time and then an absolute value of a difference between the full-water levels H2 and H3 is less than a designated value Hd, the water feeding is completed and then if it is higher than Hd, occurrence of abnormal (E4) of set water level is displayed at a character display lamp so as to warn an abnormal state at the setting of full water level.