1. A lymph node detecting apparatus comprising: an excitation light source unit, illuminating excitation light onto a living body observation portion that includes a lymph node near a tumor into which a fluorescent dye that emits fluorescence of a predetermined wavelength has been injected in advance; an optical filter, transmitting a fluorescence image generated from the living body observation portion; an image pickup device, picking up the fluorescence image transmitted through the optical filter; an adjusting means, adjusting at least one of a luminance and a contrast of an observation image output from the image pickup device; and an image displaying means; displaying the observation image, adjusted by the adjusting means, as an image for detecting the lymph node, wherein the optical filter transmits simultaneously, in addition to the fluorescence image, at a predetermined light intensity, a reflection image from the living body observation portion illuminated by the excitation light, and the observation image, in which a fluorescence picture image that corresponds to the fluorescence image and a normal picture image that corresponds to the reflection image of the excitation light are overlapped, is obtained by the image pickup device, the excitation light source unit has a plurality of excitation light sources and a supporting plate, on one surface of which the plurality of excitation light sources are installed, each of the plurality of excitation light sources is a light source that emits, as the excitation light, light of the same wavelength, and the plurality of excitation light sources are arrayed two-dimensionally with a central axis of the excitation light source unit, which is matched with an optical axis of the image pickup device, as a symmetry axis.