PURPOSE: To obtain good quality discharge water by eliminating the discharge of sewage in an aeration non-treated state, by arranging a discharge pipe with a float, which is constituted in a freely extendible manner and controls the discharge of sewage flowed in so as to exceed a predetermined volume, in an aeration tank.

CONSTITUTION: Sewage is aerated by air introduced by the operation of a blower 6 in an aeration tank 5 to which a part of the sludge from a precipitation tank 7 containing aerobic microorganisms in a multi-stage fashion is returned and mixed with sludge to be flowed into the precipitation tank 7 while receives adsorptive oxidation. A discharge pipe 8 with a float is arranged in the aeration tank 5 in a state constituted in a freely extensible manner. The base 81a of a pipe main body 81 is connected to the discharge port 51 of the aeration tank 5 in order to be capable of storing sewage flowed in from a sewage source 1 or a conditioning tank 2 to the predetermined volume of the aeration tank 5 while a slide pipe 82, of which the diameter is larger than that of the pipe main body 81, is inlaid with the pipe main body 81 in a freely extensible manner.