The invention provides a metal salt of a crosslinked cellulose derivative represented by the following formula (I), wherein the degree of substitution of the hydroxyl group of glucose unit of the crosslinked cellulose derivative by a functional group a is 1 or more. R—O—A  (I) {In the formula (I), R represents a crosslinked cellulose residue and A represents a functional group a having cation-exchange ability.}