A wastewater treatment method for an activated sludge treatment of raw water 1a by a sequencing batch reactor treatment method, the said sequencing batch reactor treatment method being a method in which: raw water 1a is introduced into a first sequencing batch reactor 2a and a second sequencing batch reactor 3a, each of which is equipped with an aeration device, a stirring device and a drainage device, and is subjected to activated sludge treatment therein; the sludge precipitated on the bottom of both the sequencing batch reactors is withdrawn (Xa) and transferred into a first excess sludge tank 8a, and then is thickened and stored in a second excess sludge tank 9a, and discharged; and the supernatant liquid in the sequencing batch reactors is discharged into a river, the said wastewater treatment method comprising: (i) using a sludge retention tank 12a equipped with an aerator and a stirrer and a thickened sludge retention tank 13a equipped with an aerator and a stirrer as the first excess sludge tank 8a and the second excess sludge tank 9a, respectively; (ii) performing a first sludge returning step Va that is a step of returning sludge to the first sequencing batch reactor 2a and the second sequencing batch reactor 3a from the sludge retention tank 12a and the thickened sludge retention tank 13a in both of which the sludge was previously aerated and stirred; and (iii) performing a second sludge returning step Wa that is a step of returning sludge, which was previously aerated and stirred in the thickened sludge retention tank 13a, to the sludge retention tank 12a; wherein the sludge in the thickened sludge retention tank 13a is supplied from the sludge retention tank 12a through a sludge thickening step Za, thereby maintaining the number of genus Bacillus bacteria in the sequencing batch reactors 2a and 3a. raw water 1a is introduced into a first sequencing batch reactor 2a and a second sequencing batch reactor 3a, each of which is equipped with an aeration device, a stirring device and a drainage device, and is subjected to activated sludge treatment therein; the sludge precipitated on the bottom of both the sequencing batch reactors is withdrawn (Xa) and transferred into a first excess sludge tank 8a, and then is thickened and stored in a second excess sludge tank 9a, and discharged; and the supernatant liquid in the sequencing batch reactors is discharged into a river, the said wastewater treatment method comprising: (i) using a sludge retention tank 12a equipped with an aerator and a stirrer and a thickened sludge retention tank 13a equipped with an aerator and a stirrer as the first excess sludge tank 8a and the second excess sludge tank 9a, respectively; (ii) performing a first sludge returning step Va that is a step of returning sludge to the first sequencing batch reactor 2a and the second sequencing batch reactor 3a from the sludge retention tank 12a and the thickened sludge retention tank 13a in both of which the sludge was previously aerated and stirred; and (iii) performing a second sludge returning step Wa that is a step of returning sludge, which was previously aerated and stirred in the thickened sludge retention tank 13a, to the sludge retention tank 12a; (i) using a sludge retention tank 12a equipped with an aerator and a stirrer and a thickened sludge retention tank 13a equipped with an aerator and a stirrer as the first excess sludge tank 8a and the second excess sludge tank 9a, respectively; (ii) performing a first sludge returning step Va that is a step of returning sludge to the first sequencing batch reactor 2a and the second sequencing batch reactor 3a from the sludge retention tank 12a and the thickened sludge retention tank 13a in both of which the sludge was previously aerated and stirred; and (iii) performing a second sludge returning step Wa that is a step of returning sludge, which was previously aerated and stirred in the thickened sludge retention tank 13a, to the sludge retention tank 12a; wherein the sludge in the thickened sludge retention tank 13a is supplied from the sludge retention tank 12a through a sludge thickening step Za, thereby maintaining the number of genus Bacillus bacteria in the sequencing batch reactors 2a and 3a.