A septic tank includes a chamber, an inlet through which waste water enters the chamber, an outlet located below the inlet, and an outlet pipe including an outlet section extending through the outlet. The outlet pipe further includes an inclined section in the chamber and extending downward from the outlet pipe. The inclined section includes a closed lower end. The inclined section further includes a suction portion located below the outlet. The suction portion includes a plurality of through-holes in a circumference thereof for sucking dischargeable purified waste water in the chamber into the inclined section and then discharged via the outlet section.