To wash a filter medium continuously by a method in which in equipment which grows a flock which was grown by adding a coagulant into a filler accumulation zone in a stagnation part and separate it, the filler, after being taken out intermittently from the lower part of the accumulation zone and washed, is supplied to the upper side of the accumulation zone.

When raw water added with an inorganic coagulant is supplied from a raw water inflow pipe 6 to a baffle plate 9, the formation of a flock of suspended substances occurs. Among flocks formed, a flock having a sedimentation velocity higher than a raw water passing velocity is precipitated to be accumulated in the lower part of a tank 1, and other flocks flow through a filler accumulation zone 22 in an ascending current. In the accumulation zone 22, water of the ascending current flows through a filler 11, in the cylinder of the filler 11, and others, and most flocks the diameter of which increases are precipitated. By the upward flow of the filler 11 in a suction tube 3, washing is carried out in the expansion part 30 of a filler supply part 2, the filler 11 which was made clean is repacked.