1. A lymph node detecting apparatus comprising: an excitation light source, 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 means, 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 means; and an image displaying means; displaying the observation image, adjusted by the adjusting means, as an image for detecting the lymph node.