Using the optical method developed in this study and based on the absorption of light from an external reference source by a solid (suspended particles), the concentration profile of settling particles along the flow of a suspension through an extended "transparent" system that model deep-bed filtration with allowance for gravity has been experimentally determined. At a constant differential pressure across the inlet and outlet of the filtration cell, a flow rate of the suspension over time has been measured. By using methods of continuum mechanics, the continuity equation for a selected moving volume of the suspension has been derived, with the change in mass of particles in the volume being proportional to the concentration in unit volume of the porous membrane filter with variable porosity. From the concentration of particles deposited along the depth of the filter, the proportionality factor for the proposed relation of the rate of deposition of particles on the porous skeleton to the concentration of particles in the suspension has been found.