A coagulation-flocculation apparatus for conducting coagulation-flocculation treatment by adding a coagulant or flocculent to suspension such as sludge is capable of diffusing the flocculant into the suspension uniformly so as to exhibit flocculation function of the flocculant effectively, thereby enhancing the flocculation efficiency, shortening the processing time, and allowing reduction in size of the apparatus. The flocculation apparatus has transfer piping 12 for transferring suspension, small-diameter piping 13 of which diameter is smaller than that of the transfer piping 12 and which is connected to the transfer piping 12, a flocculent supplying device disposed on the small-diameter piping 13 or the transfer piping 12 on the upstream side thereof, and a temporary retention section 11 for flocculation having an inlet and an outlet for water, the small-diameter piping 13 being connected to the inlet. The cross-sectional area of flow path at at least a part of the temporary retention section 11 in the direction from the inlet toward the outlet is larger than the cross-sectional area of the transfer piping 12.