Observation is performed at a more appropriate sensitivity without reducing an image quality. Provided is a fluorescence observation device (1) including: an excitation light source (3) that emits excitation light to be radiated onto a subject (A) a fluorescence-image acquiring section (21) provided with an imaging element (18) that acquires a fluorescence image (G2) by imaging fluorescence produced in the subject (A) when the excitation light emitted from the excitation light source (3) is radiated onto the subject (A) and a sensitivity adjusting section (22) that adjusts, based on luminance information of the fluorescence image (G2) acquired by the imaging element (18) of the fluorescence-image acquiring section (21), a number of pixels for binning summing (B) and/or an exposure time (t) in the imaging element (18) such that a SN ratio of the fluorescence image (G2) is equal to or larger than a predetermined threshold.
