PURPOSE: To normally form a filter bed in which compactness necessary for filtration is maintained by installing compacting pipes, a filter medium feeding conveyor, and a baffle plate and a filter medium withdrawal conveyor in the upper part of a tank body, in the central place where the conveyor is embedded in the filter medium, and in the lower part respectively.

CONSTITUTION: A tank body 1 packed with a filter medium is provided with a raw water inflow pipe 2 having a baffle plate 3. In the upper part of the tank body 1, plural compacting pipes 10 having a fixed inclination are installed to maintain the compactness of the filter medium for increasing the filtration accuracy. And below the pipes 10, a filter medium feeding conveyor 9 is installed in a place where it is embedded in the filter medium. In the lower part of the tank body 1, a baffle plate 4 is installed to reduce the variation in the compactness of the filter medium associated with the movement of the filter medium. And in the utmost lower part, a filter medium withdrawal conveyor 5 is installed. In such a way, since after the filter medium is surely and continuously transferred by a fixed quantity and is washed with pressure water from a washing nozzle in a washing tank 6, it is fed to the tank body 1 again, the filter bed is normally maintained in a stable state.