PURPOSE: To eliminate the obstraction of the slide-down of a precipitate due to an obliquely rising stream, by precipitating solid substance on the surface of a perforated inclined plate when water containing said solid substance rises obliquely and falling the same into a separation chamber to eliminate the re-entry of the precipitate into water.

CONSTITUTION: When water containing solid substance sent into from a raw water inlet 5 rises obliquely through a gap 7, the solid substance is precipitated onto the surface of a perforated inclined plate 2 and enters a separation chamber 10 from orifices 8. The upper end of the separation chamber is closed by a closing plate 4 and water is not flowed upwardly. Water is obliquely raised to be flowed into a treating tank 9 and discharged from a discharge pipe 21. The solid substance is transferred to the bottom 11 of the separation chamber 10 and transferred to a thickener bottom 15 through a falling pipe 14 from an opening part 13 to be discharged through a valve 16. Plural falling pipes 14 are studded in a bottom part 6 but, because gaps are mutually provided between pipes, raw water is extended throughout the cross area of the tank bottom part and is obliquely raised through all gaps 7 between separation chambers 10 to carry out solid substance precipitating and separating work. Raw water rising from the tank bottom part is not collided with the precipitate solid down toward the lower end of the inclined plate.