Abstract: A process for treating wastewater including: feeding organic wastewater to a first aerobic tank and an anoxic tank; nitrifying and denitrifying the organic wastewater by bacteria supported on carriers in the first aerobic tank followed by discharging primary treated water; denitrifying the organic wastewater and the primary treated water by bacteria in an activated sludge in the anoxic tank followed by discharging secondary treated water; nitrifying the secondary treated water by bacteria in an activated sludge in the second aerobic tank followed by discharging tertiary treated water; and separating the sludge from the tertiary treated water in the solid-liquid separator followed by discharging final treated water while returning the sludge to the anoxic tank. According to this process, a load in a latter anoxic tank can be reduced, the anoxic tank can be downsized compared with the prior process and treated water with good water quality is stably provided.