An object of the invention is to provide an environmentally-responsive dye-accumulated nanoparticle for grasping an intracellular environment such as pH with respect to each cell. The invention provides an environmentally-responsive dye-accumulated nanoparticle, in which an environmentally-responsive dye is accumulated on the surface of, or inside a thermosetting resin particle, and which has an average particle diameter of 30 to 800 nm, wherein the accumulated amount of the environmentally-responsive dye is 200 to 10,000,000 molecules/particle. The environmentally-responsive dye has dependence on pH, or a Ca ion in terms of fluorescence intensity, and the thermosetting resin is a melamine resin.
