A fluorescence observation device, comprising: an illumination section configured to radiate excitation light and reference light onto an object; a fluorescence-image acquisition part configured to acquire 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 configured to acquire 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 configured to correct 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 configured to set 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 configured to extract 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 configured to display the region of interest extracted by the extraction part and the reference image in association with each other; and an input section to input a determination result as to whether the region of interest displayed on the display section is a desired region with a suspected lesion or not, 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 input section, wherein the threshold setting part sets one of the first coefficient and the second coefficient to a fixed value and (i) maintains or reduces the other coefficient when a result indicating that the determination result that the extraction part extracted the desired region as the region of interest is input through the input section; and/or (ii) increases the other coefficient when a result indicating that the determination result that the extraction part did not extract the desired region as the region of interest is input through the input section. an illumination section configured to radiate excitation light and reference light onto an object; a fluorescence-image acquisition part configured to acquire 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 configured to acquire 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 configured to correct 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 configured to set 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 configured to extract 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 configured to display the region of interest extracted by the extraction part and the reference image in association with each other; and an input section to input a determination result as to whether the region of interest displayed on the display section is a desired region with a suspected lesion or not, 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 input section, wherein the threshold setting part sets one of the first coefficient and the second coefficient to a fixed value and (i) maintains or reduces the other coefficient when a result indicating that the determination result that the extraction part extracted the desired region as the region of interest is input through the input section; and/or (ii) increases the other coefficient when a result indicating that the determination result that the extraction part did not extract the desired region as the region of interest is input through the input section. (i) maintains or reduces the other coefficient when a result indicating that the determination result that the extraction part extracted the desired region as the region of interest is input through the input section; and/or (ii) increases the other coefficient when a result indicating that the determination result that the extraction part did not extract the desired region as the region of interest is input through the input section.