Abstract: The invention relates to a continuous and controlled filter device that filters gases and liquids while maintaining a low differential pressure across the filter element. The device automatically advances clean filter medium into the filter seat or window when the differential pressure rises to a predetermined level replacing dirty filter medium with clean filter medium. The advance of the filter medium is effected by a fluid pressure operated expansible device cooperating with an indexing device for positioning the filter medium as a function of the pressure differential extending thereacross. The indexing device may include a ratchet structure for positively positioning preselected portions of the filter medium across the flow passage through which the fluid being filtered is passed.