A fluorescence observation apparatus comprising: an excitation light source configured to emit excitation light to be radiated onto an imaging subject; a fluorescence-image acquisition portion that is provided with an imaging device configured to acquire a fluorescence image (G 2) by capturing fluorescence generated at the imaging subject due to the irradiation with the excitation light emitted from the excitation light source; and a sensitivity adjusting portion configured to adjusts a number of pixels for binning summing and optionally an exposure time for the imaging device on the basis of luminance information of the fluorescence image acquired by the imaging device of the fluorescence-image acquisition portion so that signal-to-noise ratio of the fluorescence image becomes equal to or higher than a predetermined threshold, characterized in that the sensitivity adjusting portion includes: calculate an incident-light-level calculating portion configured to calculate an incident light level on the imaging device based on the luminance information; and a storage portion configured to store a correspondence relationship between the incident light level on the imaging device and the number of pixels for binning summing, and wherein the storage portion is configured to store a correspondence relationship between the incident light level on the imaging device and the SN ratio for each number of pixels for binning summing, wherein the number of pixels for binning summing is determined based on the correspondence relationship between the incident light level on the imaging device and the number of pixels for binning summing and the correspondence relationship between the incident light level on the imaging device and the SN ratio for each number of pixels for binning summing stored in the storage portion by using the incident light level calculated by the incident-light-level calculating portion. an excitation light source configured to emit excitation light to be radiated onto an imaging subject; a fluorescence-image acquisition portion that is provided with an imaging device configured to acquire a fluorescence image (G 2) by capturing fluorescence generated at the imaging subject due to the irradiation with the excitation light emitted from the excitation light source; and a sensitivity adjusting portion configured to adjusts a number of pixels for binning summing and optionally an exposure time for the imaging device on the basis of luminance information of the fluorescence image acquired by the imaging device of the fluorescence-image acquisition portion so that signal-to-noise ratio of the fluorescence image becomes equal to or higher than a predetermined threshold, characterized in that the sensitivity adjusting portion includes: calculate an incident-light-level calculating portion configured to calculate an incident light level on the imaging device based on the luminance information; and a storage portion configured to store a correspondence relationship between the incident light level on the imaging device and the number of pixels for binning summing, and calculate an incident-light-level calculating portion configured to calculate an incident light level on the imaging device based on the luminance information; and a storage portion configured to store a correspondence relationship between the incident light level on the imaging device and the number of pixels for binning summing, and wherein the storage portion is configured to store a correspondence relationship between the incident light level on the imaging device and the SN ratio for each number of pixels for binning summing, wherein the number of pixels for binning summing is determined based on the correspondence relationship between the incident light level on the imaging device and the number of pixels for binning summing and the correspondence relationship between the incident light level on the imaging device and the SN ratio for each number of pixels for binning summing stored in the storage portion by using the incident light level calculated by the incident-light-level calculating portion.