A flush mixing tank is provided with a flocculant mixing section in which a raw water a to be treated is introduced and a flocculant such as known aluminum sulfate or polychlorinated aluminum is injected, and then they are agitated and mixed; and a floc growing section through which flocculant injected water mixed with the flocculant passes, and the flocculant injected water is discharged as a floc treated water, but a water passing member, which is provided with a plurality of water through-holes for promoting the growth of the flocs in the flocculant injected water, is disposed in the floc growing section. Flocs of flocculant injected water can be rapidly formed, and growth of the flocs can be accelerated.