The separation-permeation performance of porous silicon nitride (Si3N4) ceramics (consisting of columnar grains connected at random in three dimensions) as membrane filters was evaluated, and compared with commercial Al2O3 membranes having a three-layer structure. Si3N4 membranes separate particles with diameters much less than their pore diameters. The permeability of Si3N4 membranes with separability values the same as those of the Al2O3 membranes was about 1.3-2.4 times as large as the Al2O3 membranes. Dead-end filtration examination, using Al2O3 particles with a particle size distribution, indicated that the Si3N4, membrane filtration mechanism obeyed the cake filtration mechanism although the particle size was smaller than the pore size of the Si3N4 membranes.