The membrane filters are composed of cellulose ester such as cellulose acetate, cellulose butyrate, cellulose acetate bytyrate (ester mixture) or cellulose nitrate, cellulose ether such as ethyl cellulose, carbonate ester polymer such as polycarbonate of bisphenol A, polyamide such as polycapramide (6-nylon) or polyhexamethyleneadipamide (6,6-nylon), or the like.