1. A fluoroscopy apparatus comprising: a light source unit configured to generate illumination light and excitation light that are radiated onto an imaging subject; a fluorescence-image generating unit configured to generate a fluorescence image by imaging fluorescence generated at the imaging subject as a result of being irradiated with the excitation light from the light source unit; a return-light-image generating unit configured to acquire returning light from the imaging subject irradiated with illumination light from the light source unit and generate a return-light image; an image correcting unit configured to generate a corrected fluorescence image by dividing the signal strengths of pixels in the fluorescence image generated by the fluorescence-image generating unit by signal strengths of pixels in the return-light image generated by the return-light-image generating unit; a characteristic-value extracting unit configured to extract a characteristic value of the corrected fluorescence image generated by the image correcting unit; a threshold determining unit configured to determine a threshold for discrimination between a region of interest and another region of the corrected fluorescence image using a history of characteristic values extracted by the characteristic-value extracting unit; and a discriminating unit configured to discriminate between the region of interest and another region in the corrected fluorescence image on the basis of the threshold determined by the threshold determining unit.