PURPOSE: To perform sepn. effectively in a separating method for colloidal materials in liquid by combining membrane sepn. and settling separation.

CONSTITUTION: Liquid (a) to be treated contg. colloidal materials enters a settling separator 1, where while the materials stagnate for a specified time, the colloidal materials (e) having relatively large particle sizes are separated and liquid (b) to be treated is obtained. The liquid (b) is pumped through the discharge outlet of the separator 1 into a separator 3 consisting of a permselective membrane, where the liquid is separated to membrane permeated liquid (f) contg. no colloidal materials and concd. liquid (c) wherein the colloidal materials of relatively small diameters are concd. After the liquid (c) is discharged from the separator 3, the liquid is pumped 4 into a settling separator 5, where the greater part of the colloidal materials (g) of relatively small particle sizes are separated in the same way as mentioned above, the liquid (d) to be treated is obtained. The liquid (d) is mixed with the liquid (b) with a pump 6 and the mixture is again treated in the separator 3. On the other hand, the materials (e), (g) separated with the separators 1, 5 are discharged from the discharge ports of the respective devices.