A flocculation device includes: a high rate agitation tank which generates magnetic micro flocs in a treated water by agitating the treated water to which magnetic particles and a flocculant are added, with a agitating blade rotating at a first rotating speed; a decelerating vessel which reduces a speed of the treated water flowing out of the high rate agitation tank to a predetermined speed and in which a polymer flocculant is added to the treated water; and a low rate agitation tank which generates magnetic flocs in the treated water by agitating the treated water flowing out of the decelerating vessel with a agitating blade rotating at a second rotating speed which is slower than the first rotating speed. In addition, the high rate agitation tank, the decelerating vessel and the low rate agitation tank are integrally incorporated and filled with the treated water.
