During a waste water treatment process such as an activated-sludge process using microorganisms, the present invention ozonizes a microorganism-mixed liquid. It reduces the amount of microorganisms remaining, prevents a microorganism floc from sedimenting inappropriately, and recovers appropriate sedimentation while maintaining treatment performance. A waste water ozonization apparatus according to an embodiment of the present invention can include a sludge drawing pump which draws a microorganism-mixed liquid from an aeration vessel; an ozonizer for generating ozone gas which reacts with the microorganism-mixed liquid; an ozonization vessel which injects the ozone gas generated by the ozonizer into the drawn microorganism-mixed liquid; an ozonized-sludge channel that returns the ozonized microorganism-mixed liquid to the aeration vessel; and a control section for intermittently performing an ozonization operation.