The crossflow microfiltration characteristics of dilute suspension, which produces a highly compressible fouling cake-layer and/or a highly viscous filtrate, is studied under constant pressure. With the filtration conditions within a critical level, medium filtration at an earlier stage ceases, and crossflow microfiltration follows the principle of thin-cake filtration after operation begins. An equation for flux, which relates the cake resistance to a number of operational variables, is determined for the thin-cake filtration process by introducing a controlling parameter composed of the shear stress exerted at the cake surface and the coefficient of constant-pressure filtration. On the basis of an empirical correlation of the flux-controlling parameters, the effects of both cake-compressibility and filtrate-viscosity on the flux are evaluated.