Abstract: The invention provides an apparatus and method for plasma separation which is designed so that during the process of separating blood into plasma and corpuscular components by means of plasma separation membranes, if the rate of plasma permeation through the membranes is decreased so that the trans-membrane pressure differential exceeds a predetermined value, the number of revolutions of a plasma pump is adjusted downward to permit restoration of a trans-membrane pressure differential lower than the predetermined value. The rate of plasma discharge is gradually decreased according to the varying permeability of the membranes to plasma, whereby a most effective performance of the membranes can be assured. The apparatus and method can be employed in practicing various blood treatment methods wherein blood is separated into plasma and corpuscular fractions to remove the plasma containing pathogenic factors or to remove the pathogenic factors from the plasma.