1. A fluorescence observation apparatus comprising: a light source configured to radiate illumination light and excitation light onto a subject; and a processor comprising hardware, wherein the processor is configured to: generate 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; generate a fluorescence image in which fluorescence emitted from the subject due to the irradiation with the excitation light from the light source is captured; perform a detection of a fluorescence region having gradation values equal to or greater than a predetermined gradation value threshold in the fluorescence image; adjust gradation values of the return-light image; generate a superimposed image in which the return-light image, in which the gradation values have been adjusted, and the fluorescence image are superimposed; and set a degree-of-reduction, by which the gradation values of the return-light image are adjusted, based on a detection result of the detection of the fluorescence region, wherein, in the case in which the fluorescence region is detected, set 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.