PURPOSE: A multi-stage sewage clarification system using high-concentration oxygen wherein the clarification efficiency is improved by enhancing the utility factor of dissolved oxygen in a solution and reducing excess of oxygen.

CONSTITUTION: A first gas-liquid contactor 151 is fed with supernatant from a feed pipe 24 and with an oxygen-rich gas from a feed pipe 16, and the resulting oxygen- rich solution is passed through a pipe 25 for mixing with sewage fed from a sewage feed pipe 28. With the mixture fed to a disffuser pipe 6, the sewage is released into a high-density sludge blanket 4 in a lower-stage treatment tank 1 and clarified while rising into an upper-stage treatment tank 2. The tank 2 is fed with a mixture of a gas (from an accumulator 17) and supernatant through a second gas-liquid contactor 152 and a diffuser pipe 7, so that the sewage is clarified while rising through a sludge blanket 5, and the supernatant is discharged via a pipe 13.