The present invention provides a fluorescent label that can be used for carrying out a biological substance detection method for specifically detecting a biological substance from a pathological specimen, by which method, when immunostaining using a fluorescent label and staining for morphological observation using a staining agent for morphological observation are simultaneously performed, the results of fluorescence observation and immunostaining can be assessed properly even if the fluorescent label and/or the staining agent is/are deteriorated by irradiation with an excitation light. The fluorescent label is a fluorescent dye-containing nanoparticle in which the parent material is a cross-linked polymer and the fluorescent dye is an aromatic ring-based dye molecule. The cross-linked polymer is suitably a melamine resin or a styrene resin. The aromatic ring-based dye molecule is suitably a perylene and more suitably a perylene diimide. The dye molecule can have a polar group such as sulfonic acid group or its acid halide.