To provide an apparatus for treating waste water which treats waste water without making harmful chlorine-based gas leak to the outside and dispenses with the replacement and maintenance of the agitation unit thereof and to provide a method for treating waste water.

The apparatus for treating waste water is provided with: a waste water treatment tank 3 for treating waste water; a waste water supplying pipe 2 for supplying waste water to the waste water treatment tank 3; an agitating means 4 which is connected to side face near the bottom on the outside of the waste water treatment tank 3 or arranged near the upper part of the waste water treatment tank 3 and which supplies a chemical capable of decomposing dissolved matter contained in waste water to the waste water sucked from the vicinity of the bottom of the waste water treatment tank 3 and discharges the dissolved matter-decomposed treated water into the waste water treatment 3 and is used for agitating the waste water in the waste water treatment tank 3 by the energizing force of the treated water; and a treated water discharging pipe 5 for discharging the dissolved matter-decomposed treated water from the waste water treatment tank 3.