It is intended to provide asymmetric porous films which are usable in blood dialysis, plasma separation, etc. and particularly excellent in the performance of selectively separating (fractionating) plasma protein, show little endogenous coagulation, complement or quinine activily and have an extremely high biocompatibility. Porous films made mainly of a synthetic polymer and having an asymmetric structure wherein, in the sectional structure, a dense layer substantially not charged at least on the outermost surface is provided in the side on which a liquid to be treated is loaded and at least part of the film other than the outermost surface is negatively charged. In the above films, the dense layer non-charged at least on the outermost surface serves as a size barrier while the part of the film other than the outermost surface serves as a charge barrier.