The present invention addresses the problem of providing a PDD diagnostic apparatus with which, even when the distance from the optical fiber irradiation unit to the diseased tissue is increased, fluorescence detection sensitivity equal to when the optical fiber irradiation unit is brought near the diseased tissue and irradiated is obtained. The diagnostic apparatus used is provided with: a light source unit for emitting excitation light on a photosensitizing substance accumulated in the diseased tissue; a detection unit for detecting fluorescence generated from the photosensitizing substance; and a combined excitation light-irradiating / fluorescence-detecting optical fiber, to the combined exit/entrance end of which a collimator is connected. Excitation light emitted from the light source unit is guided into the combined excitation light-irradiating / fluorescence-detecting optical fiber. The excitation light emitted from the combined exit/entrance end is irradiated in a collimator-collimated state and excites the photosensitizing substance accumulated in the diseased tissue. Fluorescence radiated by the photosensitizing substance excited by the excitation light enters from the combined exit/entrance end via the collimator and the entering fluorescence is guided to the combined excitation light-irradiating / fluorescence-detecting optical fiber. The fluorescence generated from the photosensitizing substance is detected in the detecting unit.