To provide an aggregation reactor capable of ensuring a high aggregation reaction efficiency in abroad flow rate range and of expanding the range of an amount treated.

In the aggregation reactor 1 having a transfer pipe 4 feeding by pressure suspension water, a small-sized pipe, which is connected to the transfer pipe 4, smaller than the transfer pipe 4 in diameter, a flocculant feeding mean installed to the smaller-diameter-pipe or the transfer pipe 4 of the upper part and an aggregation reaction vessel 2 of which the smaller-diameter-pipe is directly opened, a plurality of large and small different diameter smaller-diameter-pipes (smaller-diameter-pipes 5A, 5B) are provided, and valves V1, V2 are provided to the respective smaller-diameter-pipes 5A, 5B.