An external device for cleaning a filtration material is applied to a filtration device for filtering a supplied unfiltered liquid by layers of a filtration material to remove muddiness in the liquid and discharging the cleaned liquid to the outside. The cleaning device has an upstanding outer tube, a screw conveyor rotatably placed in the outer tube and transferring the filtration material upward from the lower rod to the upper end of the outer tube while rub-washing the filtration material, a drive section for rotationally driving the screw conveyor, a control section for controlling rotation of the drive section, and a discharge tube for discharging the filtration material having been rub-washed, into a filtration container from the inside of the outer tube and an introduction tube that introduces the filtration material into the outer tube.