Abstract: A nutrient solution of a microorganism to be supplied to a fermentor or a culture broth in the fermentor is aseptically filtered by a porous filter medium made of polytetrafluoroethylene polymer and having properties impervious to microorganism, resistant to heat and chemicals, and hydrophilic, wherein said hydrophilic property was given to the surface of said filter medium with a treatment for hydrophilicity. This treatment comprises treating polytetrafluoroethylene porous polymer as a raw material for the filter medium with surface treating agent comprising an alkali metal naphthalene and diamine and immersing the treated polymer into concentrated sulfuric acid containing an aliphatic or aromatic aldehyde dissolved therein in a concentration exceeding half saturation.