To provide a horizontal flocculator with no segmentalization of agitator blades even in a long apparatus, and keeping a high agitation efficiency over the whole range of a flocculation basin.

The horizontal flocculator installed in a water treatment tank 10 has a drive device 16 disposed outside the water treatment tank, and an agitation blade frame 12 disposed inside the water treatment tank 10. The agitation blade frame 12 has a horizontal axis, one or more agitation blades 26 axially extended in parallel, and a support structure 14 rotatably supporting the agitation blade frame 12 around the axis. The support structure 14 supports the agitation blade frame 12 at only an outer periphery part of the agitation blade frame 12, at the portions except both ends of the agitation blade frame 12.