Disclosed is a metal salt of a crosslinked cellulose derivative represented by the general formula (I) below, wherein the substitution degree of hydroxy groups in the glucose units of the crosslinked cellulose derivative by a functional group a is not less than 1. 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.)
