An apparatus for purifying polluted water is disclosed, which can naturally remove polluted water stagnated in a treatment tank. The apparatus includes a treatment tank and an infiltration tank. The treatment tank has an inlet at its one side, and first and second outlets at its other side. The treatment tank treats the polluted water that flows through the inlet and discharges the treated polluted water through the first outlet. The infiltration tank is connected to the second outlet and has a plurality of infiltration holes formed on the body thereof. The infiltration tank allows the treated polluted water that flows from the second outlet of the treatment tank to permeate into the ground though the plurality of infiltration holes. Since the polluted water stagnated in the treatment tank can be naturally released, odor due to decay of the polluted water and a subsequent insect infestation can be prevented. In addition, maintenance fees to maintain to the treatment tank can be reduced.