To provide a wastewater treating method efficiently removing organic matter, nitrogen and phosphorus in wastewater.

Nitrogen and phosphorus in wastewater are removed by intermmitent aeration in two serially connected aeration tanks of a first aeration tank 10 and a second aeration tank 20. Then, wastewater in the second aeration tank after the intermittent aeration is introduced to a membrane separation device 30 and separated into treated water and thickened sludge, and the thickened sludge is returned to the first and second aeration tanks 10 and 20. The return amount to the first aeration tank 10 is set less than that to the second aeration tank 20.