To provide an external filter medium-washing device for filtering devices which makes easy and automatic washing of a filter medium possible at a low cost but does not require the replacement of a filtering device used at present with a newly bought one.

The external filter medium-washing device 1a is a washing device 1a applied to a filtering device 100 which filtrates a supplied raw liquid through the layer 112 of a filter medium 112a to remove contaminants from the raw liquid and discharges the clarified liquid outside. In addition, the washing device 1a is provided with: an upright external cylinder 2'; a screw conveyor 4' which is rotatively arranged in the external cylinder 2' and conveys the filter medium 112a upward, from the lower end part of the external cylinder 2' to its upper end part, while washing the filter medium 112a by rubbing it; a driving part 6 which rotatively drives the screw conveyor 4'; a control part 30 which controls rotation by the driving part 6; and a discharging pipe 50a which discharges the filter medium 112a, which is washed by rubbing, into a filtration tank 101 from an introduction pipe 38a, which introduces the filter medium 112a into the external cylinder 2', and from the external cylinder 2'.