A sentinel lymph node detecting apparatus 1 according to an embodiment of the present invention includes: a light source unit 2, illuminating an excitation light 10 and an illumination light 11 onto a biological observation location 20 that includes a sentinel lymph node 21 near a tumor, into which a fluorescent dye that emits fluorescence is injected; an optical filter 3, transmitting a fluorescence image 11 and a normal image 13; an image pickup device 5, disposed integral to the light source unit 2 and picking up the fluorescence image 11 and the normal image 13 transmitted through the optical filter 3; and an image display device 7, displaying the observation images that have been picked up. The wavelength of the illumination light 11 is set to a value close to the wavelength of the fluorescence emitted from the fluorescent dye.