To realize a water cleaning method and a water cleaning system capable of eliminating the excess and deficiency of a dissolving amount of ozone and capable of simplifying a control mechanism by detecting the concentration of ozone gas generated from water in which ozone is dissolved and controlling the amount of ozone dissolved in water corresponding to the detection result.

An ozone sensor 81 sends a detection signal of a detection value corresponding to the concentration of ozone of ozone gas 43 to an ozone gas monitor 80. The detection signal is converted to a digital value by an A/D converter circuit 87 to be taken in a CPU 83 through an input/output circuit 86. The CPU 83 operates the concentration N of the ozone gas based on the taken-in digital value and sends a start signal to an ozonizer 70 when the operated value is not a reference value N1 or more. The ozone gas formed in the ozonizer 70 is diffused in the waste water 31 stored in an ozone aeration tank 30 through an air diffusing pipe 15 to sterilize the waste water 31.