1. A fluorescence observation apparatus comprising: an illuminator that is adapted to be inserted into a body cavity of a biological organism and that is adapted to emit excitation light onto an inner wall of the body cavity; an image acquirer that acquires an image of fluorescence generated by the excitation light emitted from the illuminator so as to acquire image information; and a processor, the processor programmed to count the number of fluorescence generation sites, which are sites existing in the inner wall of the body and are generating fluorescence, having at least a predetermined fluorescence intensity on the basis of the fluorescence intensity included in the image information acquired by the image acquirer, the fluorescence generation sites being generated when a fluorescent probe whose fluorescence characteristic changes by reacting with a predetermined molecule present in the inner wall of the body cavity is excited or when a fluorescent probe whose fluorescence characteristic changes by reacting with a product produced by activation of the molecule is excited.