The crosslinked poly(acrylic acid) (salt) polymer in the present invention is a polymer which is obtained by a process including the step of polymerizing a monomer including acrylic acid and/or its salt in an amount of favorably 50 to 100 mol %, more favorably 70 to 100 mol %, still more favorably 90 to 100 mol %, and has a crosslinked structure in its inside.