PURPOSE: To always obtain treated water of high clarification degree without whirling and channeling phenomena in a sludge bracket (SB) layer by placing agitating means rotating in the layer in a treating vessel and by making the agitating force of the means smaller than that of agitating vanes.

CONSTITUTION: Flocculant-added raw water is introduced into inner cylinder 13 through introduction pipe 19, supplied to the bottom part of cylindrical space 17 through distribution pipes 20, and raised. The water undergoes the agitating action of pipes 20 and agitating vanes 18 rotating with cylinder 13 in one to flocculate SS, which is grown larger. The grown flocky SS forms SB layer S at the upper part of vanes 18, and layer S catches small flocky SS. Since layer S is agitated with weakly agitating means 21 to such a degree that pinflocks are not separated, flocks are prevented from being lumped or blocked, and treated water of high clarification degree is always obtd. from taking-out means 12.