The purpose of the present invention is to provide a water treatment system in which the amount of ozone to be supplied can be changed in accordance with the situation. The water treatment system includes: an aeration tank (1) where wastewater containing organic substances is aerobically treated to give sludge-containing treated water; an ozone supply part (2) for supplying ozone to sludge-containing treated water withdrawn from the aeration tank (1); and a bubble detection part (5) which detects bubbles generated by the reaction of the supplied ozone with the sludge. The rate of bubble amount increase, etc. detected by the bubble detection part (5) is used as an indicator for the sludge-reducing effect to control the ozone treatment.
