Oxygen permeable membrane modules are provided in a reactor in multiple stages. An oxygen-containing gas from a blower B is sequentially circulated through the oxygen permeable membrane modules via pipes and is discharged from a pipe. Raw water flows out into a bottom part of the reactor through a plurality of nozzles, and a fluidized bed F of a biological carrier such as activated carbon is formed inside the reactor. Treated water flows out of a trough via an outflow port.