To sterilize the drinking water by stably generating chlorine so as to suit to the water quality intrinsic in a seasonally changing area.

A chlorine generating electrode 6 is provided, for example, in a water reservoir 2 in a drinking water supply passage from city water and a setting instrument 12 for setting the ratio of the electric conductivity to chlorine ion concentration so as to suit to the water quality on an area in use is provided in a control part 9 for giving command to the electrode 6 to control chlorine generating concentration to be within a certain range. The residual chlorine concentration is maintained within a proper range by controlling the amt. of current for the chlorine generating electrode 6 in accordance with the area in use.