A fluorescence observation device (1, 30) comprising: an excitation light source that emits excitation light to be radiated onto a subject; a fluorescence-image acquiring section provided with an imaging element that acquires a fluorescence image (G 2) by imaging fluorescence produced in the subject when the excitation light emitted from the excitation light source is radiated onto the subject; an image quality evaluating section that calculates a fluctuation from luminance information of the fluorescence image (G 2) acquired by the imaging element of the fluorescence-image acquiring section and that calculates an SN ratio of the fluorescence image (G 2) from the calculated fluctuation and the luminance information; and a sensitivity adjusting section that adjusts a number of pixels for binning summing and/or an exposure time (t) in the imaging element such that the SN ratio calculated by the image quality evaluating section is equal to or larger than a predetermined threshold, characterized by: an illumination light source that emits illumination light to be radiated onto the subject; and a reference-image acquiring section that acquires a reference image (G 1) by imaging reflected light produced by reflection, at the subject, of the illumination light emitted from the illumination light source, wherein: - a threshold setting section (25, 26) of the fluorescence observation device (1, 30) sets the threshold based on luminance information of the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section (25, 26) adjusts the number of pixels for binning summing and/or the exposure time (t) based on the amount of blur calculated by an amount-of-blur calculating section that calculates an amount of blur from luminance informationof the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section adjusts the number of pixels for binning summing and/or the exposure time (t) based on luminance information of the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section (25, 26) adjusts the number of pixels for binning summing and/or the exposure time (t) based on the moving speed calculated by an amount-of-motion calculating section that measures the moving speed from luminance information of a plurality of reference images (G 1) acquired by the reference-image acquiring section. an excitation light source that emits excitation light to be radiated onto a subject; a fluorescence-image acquiring section provided with an imaging element that acquires a fluorescence image (G 2) by imaging fluorescence produced in the subject when the excitation light emitted from the excitation light source is radiated onto the subject; an image quality evaluating section that calculates a fluctuation from luminance information of the fluorescence image (G 2) acquired by the imaging element of the fluorescence-image acquiring section and that calculates an SN ratio of the fluorescence image (G 2) from the calculated fluctuation and the luminance information; and a sensitivity adjusting section that adjusts a number of pixels for binning summing and/or an exposure time (t) in the imaging element such that the SN ratio calculated by the image quality evaluating section is equal to or larger than a predetermined threshold, characterized by: an illumination light source that emits illumination light to be radiated onto the subject; and a reference-image acquiring section that acquires a reference image (G 1) by imaging reflected light produced by reflection, at the subject, of the illumination light emitted from the illumination light source, an illumination light source that emits illumination light to be radiated onto the subject; and a reference-image acquiring section that acquires a reference image (G 1) by imaging reflected light produced by reflection, at the subject, of the illumination light emitted from the illumination light source, - a threshold setting section (25, 26) of the fluorescence observation device (1, 30) sets the threshold based on luminance information of the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section (25, 26) adjusts the number of pixels for binning summing and/or the exposure time (t) based on the amount of blur calculated by an amount-of-blur calculating section that calculates an amount of blur from luminance informationof the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section adjusts the number of pixels for binning summing and/or the exposure time (t) based on luminance information of the reference image (G 1) acquired by the reference-image acquiring section, or - the sensitivity adjusting section (25, 26) adjusts the number of pixels for binning summing and/or the exposure time (t) based on the moving speed calculated by an amount-of-motion calculating section that measures the moving speed from luminance information of a plurality of reference images (G 1) acquired by the reference-image acquiring section.