<p>A plasma filtration method to separate blood into plasma and corpuscles with the primary filter and to remove high molecular weight substances as harmful components from said separated plasma by filtering said plasma with the secondary filter, in which an operation to open the inner chamber of the secondary filter to the atmosphere for lowering the pressure on every detection of the limit value of pressure rise caused by increase in clogging tendency in said filter and a further operation to clean said secondary filter from the sides of inner and outer chambers after lowering of pressure in the inner chamber are performed, whereby a loss of plasma caused by discharging said plasma to the outside of the system during the operation of unclogging the secondary filter can be reduced. <IMAGE></p>