A sewage treatment apparatus includes a membrane separation active-sludge treating section which performs a biological treatment on a part of sewage, which is introduced by a water introducing section while flowing through a sewer trunk line, to generate first treated sewage; a membrane highly treating section which performs a membrane high treatment on the first treated sewage to generate second treated sewage; a membrane treating tank which stores the first treated sewage; a membrane highly treating tank which stores the second treated sewage; water level sensors which respectively measure water level of the membrane treating tank and the membrane highly treating tank; and a power-control section which controls water introducing quantity of the water introducing section on the basis of the water level data measured by the water level sensors.