A polymer compound having a monosaccharide or an oligosaccharide residue, or an amino acid or peptide residue bound to a monosaccharide or an oligosaccharide residue bound to a side chain of a water-soluble polymer through a linker containing a selectively cleavable bond, the water-soluble polymer containing 20 to 80 mol % of (meth)acrylic acid residue.