1. A fluorescence observation apparatus comprising: a light source that radiates illumination light and excitation light onto a subject; a return-light-image generating portion that generates a return-light image in which return light emitted from the subject due to the irradiation with the illumination light from the light source is captured; a fluorescence-image generating portion that generates a fluorescence image in which fluorescence emitted from the subject due to the irradiation with the excitation light from the light source is captured; a fluorescence detecting portion that detects a fluorescence region having gradation values equal to or greater than a predetermined gradation value threshold in the fluorescence image generated by the fluorescence-image generating portion; a return-light-image adjusting portion that adjusts gradation values of the return-light image; a superimposed-image generating portion that generates a superimposed image in which the return-light image, in which the gradation values have been adjusted by the return-light-image adjusting portion, and the fluorescence image are superimposed; and a coefficient setting portion that sets a degree-of-reduction, by which the gradation values of the return-light image are adjusted by the return-light-image adjusting portion, based on the detection result of the fluorescence detecting portion for the fluorescence region, wherein, in the case in which the fluorescence region is detected by the fluorescence detecting portion, the coefficient setting portion sets the degree-of-reduction so that gradation values of the return-light image are decreased as compared with the case in which the fluorescence region is not detected by the fluorescence detecting portion.