[0030] wherein n is an integer of 2 to 500, which is prepared by the condensation reaction of amino group at ??-position of lysine with carboxyl group at ??-position of the lysine to form an amide bond, and ??-amino group of the lysine exists on the polymer chain. [0031] The water-swellable polymer gel of the present invention may contain at least one of other water-soluble polymers (hereinafter s