The present invention has an object to provide a compact aggregation precipitation method and an apparatus therefor which prevent the outflow of flocs and do not deteriorate the turbidity elimination performance even in start-up early stages. As the means to accomplish such an object, the present invention provides a method of adding an inorganic flocculant and, optionally, an organic polymer flocculent to the water to be treated and agitating the resulting mixture to effect aggregation precipitation treatment which comprises introducing the agitated solution into a first chamber of a separation tank where flocs are allowed to grow to effect solid-liquid separation and, simultaneously, the separated water is discharged out of the separation tank.