PURPOSE: To obtain the title polymer containing H, an alkyl, or the like, an alkylene, a (poly)oxide thereof or the like and H+, a monovalent ion, N+ or the like, having PO3H2 group containing grafts and having cation exchange capacity and the ability to form a complex with a divalent ion.

CONSTITUTION: An ethylenic ester of phosphoric acid (e.g. ethylene glycol methacylate phosphate) is grafted onto a raw material (e.g. polypropylene) containing R1 comprising hydrogen, an alkyl or the like and R2 comprising an alkylene, an alkylene oxide and/or an polyalkylene oxide or the like, and the grafted polymer is finished by immersion in a solution of an alkaline earth salt to obtain the purpose polymer which comprises R1 elements and R2 elements and R3+ element and R4 element which are individually a cation charged with H+, a monovalent metal ion and N+ or the like or, in combination of the same, form a divalent metal ion of R3R42+, in which several of the grafts comprise a series of groups containing a phosphate group and which has cation capacity and the ability to form a complex with a divalent metal ion.