PURPOSE: To enhance operation efficiency by certainly removing a part of an accumulated organism, by bringing pulsating raw water into contact with the excessive organism around the filter material in a container while introducing the same into the container from the outside.

CONSTITUTION: A biological oxidation apparatus is constituted of a container 2 receiving the supply of raw water 3, a porous filter material 7 which is received in the container 2 and has an organism for performing the oxidative treatment of raw water 3 adhered thereto and an air diffusing pipe 8 for feeding air into the container 2. A raw water tank 4a is provided to the biological oxidation apparatus and connected to the container 2 by a conduit 15 which is curved in a U-shape at the upper side of the container 2 and communicated with the lower end of the container 2. Therefore, raw water 3 is not introduced into the container 2 until the liquid surface level of raw water 3 introduced into the raw water tank 4a exceeds the U-shaped top part of the conduit 25 and introduced into the container 2 when said level exceeds the U-shaped top part of said conduit 25. As mentioned above, raw water 3 is supplied to the container 2 in a pulsated state.