PURPOSE: To enhance the removal capacity of an ultrafiltration membrance while the high permeation capacity thereof is held, by again filtering the permeate of the ultrafiltration membrane under pressure higher than the previous filtering pressure.

CONSTITUTION: By using an ultrafiltration membrane comprising a polysulfone polymer constituted from a dense layer having an extremely fine pore of 10W several thousands and a support layer reinforcing the same, 60l cellulose containing waste liquid containing 13,900ppm COD component is concentrated to 4l under such a condition that inlet pressure is 15kg/cm2 and outlet pressure is 0.5kg/cm2. The COD of a permeate is 6,672ppm. When this permeate is concentrated to 4l under such a condition that inlet pressure is 3.5kg/cm2 and outlet pressure is 2.5kg/cm2 by using the same ultrafiltration membrane, the COD of the permeate is 1,251ppm and the removal ratio thereof is 91%. When the second stage ultrafiltration is similarily carried out under the condition same to that of the first stage ultrafiltration, the removal ratio of COD is only 71%.