To inhibit an increase of phosphorus concentration immediately after restart of operation after interrupting the operation when wastewater is treated by an activated sludge treatment method.

In an activated sludge treatment method where the operation is interrupted when the inflow amount of raw water is decreased, and treated water is taken out after carrying out aeration for the predetermined time in an aeration tank when the operation is restarted, the concentration of phosphorus in the treated water can be controlled so as not to be increased immediately after restart of the operation, and at the same time electric power costs can be suppressed by interrupting the operation of an apparatus, such as aeration and circulation, during night and day when the inflow amount of the raw water is small.