Process for filtering plasma and regenerating a secondary filter therefor

A process for filtering plasma by separating blood from a supply source into blood cells and plasma by a primary filter, introducing the separated plasma into a secondary filter through a supply line, passing the plasma through the secondary filter from an inner chamber of the secondary filter to an outer chamber of the secondary filter to thereby filter off macromolecules from the plasma as harmful components and purify the plasma, and returning the purified plasma from the secondary filter through a return line to the supply source while mixing the purified plasma with the separated blood cells from the primary filter into blood, wherein the secondary filter is washed and regenerated by executing a pressure lowering procedure of opening the inner chamber of the secondary filter to the atmosphere every time increase of the internal pressure of the inner chamber to an upper limit value due to an increasing plugging tendency of the secondary filter is detected, and washing the interior of the secondary filter from both sides of the inner and outer chambers simultaneously with a wash liquor after the internal pressure of the inner chamber has lowered while discharging wash from the inner chamber to outside the secondary filter, the regeneration of the secondary filter being followed by a usual operation again.