1. A hydrophilic polymer having an anionic group; a polyether derivative; and a phosphor, wherein the phosphor is bound to the hydrophilic polymer via the polyether derivative, the phosphor being an indacene derivative, a fluorescein derivative, a rhodamine derivative, an indocarbocyanine derivative, a furazan derivative, a zanthene chromogen, a cyanine chromogen, a coumarin chromogen or a porphyrin chromogen; the hydrophilic polymer is a polymer in which a 3-sulfopropyl methacrylate or a 2-acrylamido-2-methylpropane sulfonate is polymerized, and wherein the anionic group is a sulfonic acid group, a phosphoric acid group, or a salt thereof and wherein the polyether derivative is a polyethylene glycol or polypropylene glycol derivative.