1. A fluorescence observation device comprising: an illumination section that radiates excitation light and reference light onto an object; a fluorescence-image acquisition part that acquires a fluorescence image by capturing fluorescence produced in the object that has been irradiated with the excitation light from the illumination section; a reference-image acquisition part that acquires a reference image by capturing return light returning from the object that has been irradiated with the reference light from the illumination section; a corrected-fluorescence-image generation part that corrects the fluorescence image acquired by the fluorescence-image acquisition part by using the reference image acquired by the reference-image acquisition part, thus generating a corrected fluorescence image; a threshold setting part that sets a gradation-value threshold based on an average of gradation values of the entire corrected fluorescence image generated by the corrected-fluorescence-image generation part, a standard deviation of the gradation values, a first coefficient related to weighting of the average of the gradation values of the entire corrected fluorescence image, and a second coefficient related to weighting of the standard deviation of the gradation values; an extraction part that determines and extracts, as a region of interest, a region in the corrected fluorescence image that has a gradation value larger than the gradation-value threshold set by the threshold setting part; a display section that displays the region of interest extracted by the extraction part and the reference image in association with each other; and a right/wrong input section through which an observer inputs a result as to whether the determination of the region of interest displayed on the display section, made by the extraction part, is right or wrong, wherein the threshold setting part sets at least one of the first coefficient and the second coefficient so as to reflect the input result input through the right/wrong input section.