To provide a filtration apparatus wherein a filter medium wound upward from a slanted plate in backwashing can easily return to a filtering layer.

In a filtering process, raw water is supplied to the lower end of a main pipe 20 via a raw water supply pipe 23. The raw water rising through the main pipe 20 enters a trough 19, overflows from both sides of the trough 19, is filtered passing through the filter layer 12 via a flow path 24, and flows out via a supply/discharge pipe 16 and a treated water taking out pipe 28 connected thereto. Most of bubbles flows through a side part flow path 30 in an air backwashing process and a gas/water combined backwashing process. A little amount of bubbles passes through the supply/discharge path 24, thus, the filter medium rising up the upper face of the slanted plate 22 drops downward through the supply/discharge path 24.