this invention relates to a filter which includes a substrate of a ceramic porous material of a high strength , which serves as a support , covered with a microporous membrane of a polymer , typically a fluorocarbon polymer , partly permeating the surface layer of the substrate and acting as a filter for fine particles . the filter exhibits excellent corrosion - resistance , durableness and heat - resistance and is used for semi - ultrafiltration such as filtration for sterilization , purification of crystals and the like .