A biological substance detection method for detecting a biological substance specifically in a pathological specimen, includes a step of immunologically staining the pathological specimen using a fluorescent label, a step of staining the pathological specimen with a staining reagent for morphology observation purposes (eosin) to observe the morphology of the pathological specimen, a step of irradiating the stained pathological specimen with excited light to cause the emission of a fluorescent and detecting the biological substance in the pathological specimen. In the step of immunologically staining the pathological specimen, a special fluorescent particle for which the excitation wavelength appears in a region that is different from the excitation wavelength region of eosin is used as the fluorescent label.