Fluorescent substance-accumulated nanoparticles and labeling agent using same

The present invention provides nanoparticles of which matrix is a thermosetting resin in which a hydrophobic fluorescent substance is accumulated, wherein the nanoparticles have luminance that is high enough to enable pathological diagnosis by using a fluorescence image of a biological substance such as a protein, a nucleic acid, or the like, labeled with the nanoparticles. In the fluorescent substance-accumulated nanoparticle according to the present invention, the thermosetting resin includes a structural unit formed from a raw material having a hydrophobic substituent, and the fluorescent substance is accumulated in the nanoparticles by at least a hydrophobic interaction, preferably additionally by a stacking interaction, occurring between the fluorescent substance and the hydrophobic substituent of the thermosetting resin.