A filtering device provided a horizontal drum in which a filter layer made of granular filter material and a space portion are formed in a vertical direction within its interior; a crude water introducing/dirty water discharging first water flow pipe which passes through an axial portion of the horizontal drum from the outside of the horizontal drum and whose a crude water introducing/dirty water discharging first water flow portion is located in the space portion; and a filtered water introducing/washing water discharging second water flow pipe which passes through an axial portion of the horizontal drum from the outside of the horizontal drum and whose a filtered water introducing/washing water discharging second water flow portion is located in an interior of the filter layer. A planar filter mesh which is rotated together with the horizontal drum and which is in communication with the filtered water introducing/washing water discharging second water flow portion is additionally provided in the interior of the filter layer.
