The present invention provides a method for removing a divalent metal cation from a macromolecular compound having an anionic functional group and containing the divalent metal cation, including (1) suspending a macromolecular compound having an anionic functional group and containing a divalent metal cation in a solution in which an electrolyte that releases an alkali metal ion is dissolved at a concentration at which the macromolecular compound is salted out, and (2) performing, in the obtained suspension, an ion exchange reaction to exchange the divalent metal cation contained in the macromolecular compound with the alkali metal ion.