A spraying means is arranged on the scum-flowing wall surface side of wall surfaces forming a scum inlet of a scum discharging mechanism. The spraying means sprays pressured water upward along the wall surface on the scum-flowing side from when a part of the scum inlet is submerged in water until the scum begins to flow into the scum inlet. As a result, even large lumps of scum exceeding 10 cm in thickness (diameter) may be lead smoothly into a pipe by the pressured water, and the amount of discharged water accompanying scum discharge may be reduced to 1/20 to 1/30 than by a conventional scum removing apparatus.