PURPOSE: To recover the flow velocity of filtration for a microporous membrane wherein the flow velocity has been lowered due to drying the microphorous membrane or making it hydroponic by immersing the microporous membrane in liquid and boiling this liquid or immersing the membrane in the liquid being boiled and thereafter intactly cooling this membrane and utilizing it without being dried.

CONSTITUTION: A microporous membrane is immersed in a solvent (e.g. water, methanol, ethanol and hexane, etc.) which does not dissolve the material of the membrane. When the solvent having low b.p. is utilized, treatment is made easier. The liquid (solvent) is boiled in this immersed state or the membrane is immersed in the liquid being boiled. Thereafter the membrane is cooled in a state wherein it has been immersed in liquid and utilized without being dried. By this method, the flow velocity of filtration for the membrane wherein the flow velocity has been lowered due to drying or to being made hydrophobic can be recovered regardless of the kind of the membrane. Furthermore the membrane and a membrane module which must be preserved in a wetted state heretofore can be preserved in a dry state. The need for a disinfectant, etc., is eliminated and therefore the amount necessary for washing can be drastically reduced at a point of time for starting utilization.