PURPOSE: To avoid an influence of air entering the inside of pipe lines, and to efficiently attain water level control, by a method wherein hot water much enough to remove the air in the pipe lines is supplied to a bathtub in cases where the detected water level is lower than a reference water level, and the hot water is supplied up to a preset water level in cases where the water level beyond the reference water level is detected.

CONSTITUTION: In the title controller using a water level-detecting means 14 for detecting the water level in a bathtub 2 by pressure, the minimum water level at which the opening part of a pipe line 8 connecting the water level-detecting means 14 goes under water in the bathtub 2 is given as a reference water level hE. When it is detected by the water level-detecting means 14 that the water level is at the reference water level hE or lower, being prior to hot-water supply, that air should enter the inside of the pipe lines 6, 8 is determined. The air is eliminated by supplementary water supply, the water level is detected again, and hot water is supplied up to a preset water level hF. In this way, in cases where the water level is beyond the reference water level hE, the water level can be controlled at the water level required by main water supply only without the supplementary water supply. Therefore, when the supplementary water supply is unnecessary, the operating mode for the supplementary water supply is omitted, and water level control is efficiently attained.