TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Property: The pore volume per unit dry weight of the porous ion exchanger is 1 ml/g or more. The exchange capacity per unit dry weight of the porous ion exchanger is 0.1 meq/g or more, where meq is milli exchange group equivalent.    TECHNOLOGY FOCUS - POLYMERS - Preferred Component: The porous ion exchanger contains one or more types of cross-linked copolymer containing structural unit of formula (1), (2) and/or (3).R1-R3,X- = same as defined above;A = 3-8C linear or branched alkylene group or 4-8C alkylene oxy methylene group; andY+ = counter cation. EXAMPLE - Styrene (in g) (8.5) and divinylbenzene (1.5) having purity of 80%, were mixed. Benzoyl peroxide (0.15) having purity of 40% was dissolved in the mixture, to obtain an oil-phase component of polymerization reaction solution. Span 80 (sorbitan monooleate) (2.8) was dissolved in water (50), to obtain water-phase component of polymerization reaction solution. The water phase and oil-phase solutions, were mixed to obtain polymerization reaction solution. The obtained solution was bubbled for 10 minutes under nitrogen stream, to remove dissolved oxygen. The obtained white emulsion was stirred for 5 minutes at 2000 rpm, heated at 90degreesC for 5 hours in a water bath, and thermal polymerization reaction was performed. After polymerization reaction, cross-linking polymer in the form of white foam was washed with desalted water, and water was removed. Nitrobenzene (200 ml) and concentrated sulfuric acid (100 ml), were added to cross-linked polymer (8), and sulfonated at 100degreesC, to obtain cation exchanger. The obtained cation exchanger was washed using water and methanol, dried at 50degreesC for 5 hours, to obtain filter medium consisting of porous cation exchanger. The obtained filter medium had exchange capacity of 0.1 meq/g and 4.2 meq/g, with respect to top row and left column, and was free from impurities. The obtained filter medium had favorable desalting property and gas collection property.