To provide a biological wastewater treatment method which enables a reduction in the volume of excess sludge and an improvement in the treated water quality, and a biological wastewater treatment apparatus.

An organic wastewater is introduced into a biological treatment tank 1, and at least either a dipping degree of an impeller 3 which is rotatable around a vertical direction axis line against an organic wastewater R or a rotation speed is controlled. Thereby, three layers consisting of an aerobic region A, a facultative anaerobic region B and an anaerobic region C are formed from the top to the bottom in the biological treatment tank 1. In accordance with these three layers A, B and C, all the biodegradable organic matters are decomposed, and at the same time, the ammoniac nitrogen generated when protein is decomposed in the aerobic region A is oxidized further to be nitrite nitrogen and nitrate nitrogen so that these nitrogen oxides can be reduced to nitrogen in the facultative anaerobic region B, and a large amount of excess sludge generated in the aerobic region A can be decomposed in the anaerobic region C, and micro-organisms with different properties can be in cooperation with one another.