PURPOSE: To purify sewage biologically efficiently by providing ≥3 connected tanks having aerators, and a final settling tank, and forming reaeration tanks which permit charging of return sludge to the 1st tank and arbitrary succeeding tanks.

CONSTITUTION: A purification plant for sewage is constituted by connecting four aeration tanks 1, 2, 3, 5 and a final settling tank 6 by means of coupling parts 21, 22, 23, 24 so that liquid flows in order of the tanks 1, 2, 3, 5, 6. Return sludge is supplied to the tank 1 through opening and closing means 15, and the tank 1 is used as a reaeration tank R which reactivates the return sludge. The raw water flowing through a line 9 into a supply line 7 is distributed and supplied into the tanks 2, 3, 5 acting as catalytic tanks through opening and closing means 12, 13, 14 which are open. The means 11, 12, 13, 14 are controlled to arbitrary openings so as to distribute the raw water into the individual tanks in the suitable required states, for example, daily changing tendencies.