Dead-end filtration experiments with semi-permeable ultrafiltration membranes were conducted by using bovine serum albumin (BSA) and dextran solutions, and the variations with time of the filtration rate and the apparent rejection of solutes by the membrane were measured during the course of filtration. It was shown that the increase in the solute rejection accelerated with the progress of filtration brought about the increase in the growth rate of the filter cake comprised of solutes rejected by the membrane. By introducing the concept of the equivalent filtrate volume per unit membrane area, the variations of filtration rate and apparent rejection were interrelated through the properties of the filter cake formed on the membrane surface, i.e., the average specific cake resistance and the average porosity of the filter cake, which were independent of the rejection properties of the membrane. Moreover, the average porosity of BSA filter cake was also evaluated by using a dead-end filter with a sudden reduction in its filtration area in filtration with semi-permeable ultrafiltration membranes.