To surely exterminate cariogenic bacteria such as Streptococcus mutans and Lactobacillus living in a biofilm by setting the pH satisfactory to both decalcification and disinfection.

The apparatus 1 for producing sterilizing water is provided with a stock solution tank 3 for storing a stock solution 2, a stroke pump 4 connected to the stock solution tank, an electrolytic bath 5 connected to the stroke pump, a discharging pipe 6 connected to the electrolytic bath, a dilution water tank 8 holding a dilution water 7, and a water level sensor 9 as a water-level measuring means to measure the water level in the diluting water tank. The setting position of the diluting water tank 8 is relatively determined based on the tip end of the discharging pipe 6 in a manner to position the tip end of the discharging pipe 6 below the water level of the diluting water 7 stored in the diluting water tank 8.