NOVELTY - A phosphorus removal apparatus includes anaerobic tanks, a settling tank, two pipes, a phosphorus-releasing tank, a solid/liquid separating device, a solubilizing device, a solubilized sludge returning device, and a phosphorus-removing device.    USE - For the biological removal of phosphorus from raw water (claimed).    ADVANTAGE - The inventive phosphorus removal apparatus can reduce the volume of sludge and increase the phosphorus removal efficiency.    DETAILED DESCRIPTION - A phosphorus removal apparatus comprises:    (a) an anaerobic tank into which the raw water and activated sludge are introduced to form a mixed liquor;    (b) an aerobic tank in which the mixed liquor from the anaerobic tank is aerated;    (c) a settling tank in which the mixed liquor flowing out of the aerobic tank is subjected to solid/liquid separation;    (d) a first pipe connecting the settling tank to the anaerobic tank where some of the sludge separated from the mixed liquor in the settling tank is returned as the activated sludge to the anaerobic tank;    (e) a phosphorus-releasing tank in which a remaining portion of the separated sludge from the settling tank is maintained anaerobically and phosphorus is released from the sludge;    (f) a solid/liquid separating device in which sludge admixed liquid from the phosphorus-releasing tank is subjected to solid/liquid separation;    (g) a solubilizing device in which sludge separated in the solid/liquid separating device is solubilized;    (h) a solubilized sludge returning device with which sludge which has been solubilized in the solubilizing device is returned to the anaerobic tank; and    (i) a second pipe for supplying some of the sludge which has been solubilized in the solubilizing device to the phosphorus-releasing tank, and a phosphorus-removing device in which phosphorus is removed from water which has been separated in the solid/liquid separating device.    DESCRIPTION OF DRAWING(S) - The figure is a schematic drawing of the invention.