NOVELTY - Sewage treatment device has a sedimentation-type solid-liquid separation tank (1) into which wastewater flows and in which the inflowed wastewater is separated into solid and liquid, an anaerobic treatment tank in which the treated water from the solid-liquid separation tank is subjected to anaerobic treatment, an aerobic treatment tank (3) in which the treated water from the anaerobic treatment tank is subjected to aerobic treatment, and a circulation return path (17).    USE - The device is useful for treatment of sewage (claimed).    ADVANTAGE - The device has high denitrification ability irrespective of the load of the inflowing wastewater.    DETAILED DESCRIPTION - Sewage treatment device has a sedimentation-type solid-liquid separation tank (1) into which wastewater flows and in which the inflowed wastewater is separated into solid and liquid, an anaerobic treatment tank in which the treated water from the solid-liquid separation tank is subjected to anaerobic treatment, an aerobic treatment tank (3) in which the treated water from the anaerobic treatment tank is subjected to aerobic treatment, and a circulation return path (17) that returns the treated water from the aerobic treatment tank to the anaerobic treatment tank. The nitrification-denitrification treatment is performed using the device by circulating activated sludge between the anaerobic treatment tank and the aerobic treatment tank. A sludge-concentration adjustment unit for adjusting concentration of sludge in the treated water transferred from the solid-liquid separation tank to the anaerobic treatment tank such that the concentration becomes suitable for the nitrification-denitrification treatment, irrespective of the load of the wastewater that flows into the solid-liquid separation tank. An INDEPENDENT CLAIM is included for method of treatment of sewage.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the sewage treatment device.    Solid-liquid separation tank (1)    Aerobic treatment tank (3)    Circulation return path (17)    Sludge storage tank (18)    Isolated sludge transfer path (21)