In order to prepare a cation exchange membrane having excellent monovalent cation permselectivity, a thin polyethyleneimine (PEI) layer was formed on one surface of the membrane by sulfonyl-amide bonding. The electrochemical properties of the cation exchange membranes, which had been prepared by the reaction of an aqueous PEI solution with chlorosulfonylated membranes, were examined in accordance with the number of the successive PEI reaction batches. The monovalent cation permselectivity decreased with the increase in the number of reaction times of the reused PEI solution. The smaller molecular weight of PEI was found to be consumed during the reaction, which means that PEI with higher molecular weight has difficulty reacting with the chlorosulfonylated membrane.