1. A fluorescence observation device comprising: a light source configured to radiate excitation light and reference light onto an object; a fluorescence capturing sensor configured to capture fluorescence produced in the object irradiated with the excitation light; a return light capturing sensor configured to capture return light from the object irradiated with the reference light; and a processor comprising hardware, wherein the processor is configured to: generate a fluorescence image based on the fluorescence captured by the fluorescence capturing sensor; generate a reference image based on the return light captured by the return light capturing sensor; generate a corrected fluorescence image by correcting the fluorescence image with the reference image; set a gradation-value threshold based on an average of gradation values of the entire corrected fluorescence image, 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; determine and extract, as a candidate region of interest, a region in the corrected fluorescence image that has a gradation value larger than the gradation-value threshold; control a display to display the candidate region of interest extracted and the reference image in association with each other; and receive a result, inputted by an observer, as to whether the candidate region of interest displayed on the display is right or wrong, wherein a result of right indicates a determination by the observer that the candidate region of interest is a region of interest, and a result of wrong indicates a determination by the observer that the candidate region of interest is not a region of interest, wherein the processor is configured to set at least one of the first coefficient and the second coefficient so as to reflect the result inputted by the observer.