1. A fluorescence observation method of observing a living organism into which a fluorescent dye is injected, the fluorescence observation method comprising the steps of: irradiating the living organism with excitation light including a wavelength for exciting the fluorescent dye using a light source; acquiring a first fluorescence image of the living organism generated by the irradiation with the excitation light using an image sensor; specifying an observation object in the living organism on the basis of the first fluorescence image; acquiring a second fluorescence image of the observation object generated by the irradiation with the excitation light using the image sensor; and specifying a linear fluorescence pattern appearing in the second fluorescence image.