We developed a novel low frequency (around 20 KHz) ultrasonic sterilization apparatus and its operation method for actual pool and spa is represented.Oscillator is attached to vibration surface as circular cone shape and, therefore,traveling wave is generated as spherical wave.The spherical wave reflects at the surface of sterilization chamber, resulting in secondary spherical wave from the surface.The power generated by cavitation, straight wave and vibration acceleration are equalized.By these reasons, it is possible to sterilize recirculating water stably by controlling frequency +/-1 KHz.10000 cfu/100 ml of Legionella pneumophila subsp.pheumophilila ATCC 33152 was used as a typical hygienic bacterium and no cfu was detected after 1 sec low frequency treatment using 150 ml glass bottel.The strain was added to 15 L of pH 9.2 of a hot spring water in a stainless chamber to become 11000 cfu/100 ml and treated at the same condition, resulting in no detection of the strain.140 cfu/ml of Escherichia coli JCM1649, 210 cfu/ml of Enterobacter faecalis JCM5803, and 190 cfu/ml of Staphylococcus aureus NBRC 100910 were respectively irradiated for 5 min.No colony detection were shown in all cases.The ultrasonic apparatus, 150 mm × 150 mm × 494 mm, 250 W/200 V, was installed after the filtration facility at the pipe line of circulation of a spa.No detection of Legionella and coli form group for initial 9 days were recorded.Monthly inspection of these bacteria for 7 years and 8 months at this time represent no detection of these bacteria.We can successfully reduce the dosage of chlorine after introducing the low frequency ultrasonic sterilization apparatus.