1. A method for generating optical tomographic information comprising: setting beforehand a diffusion coefficient and an absorption coefficient based on optical characteristics of a subject that does not itself include a phosphor and provisionally setting a concentration distribution of the phosphor within the subject, wherein the subject is a target of observation after receiving the phosphor; detecting fluorescence, having a wavelength that is set according to the optical characteristics of the subject, which is emitted from the phosphor and emitted from the subject that is the target of observation toward a periphery of the subject, by illuminating excitation light, of the wavelength that is set according to the optical characteristics of the subject, from a light source toward a surface of the subject that is the target of observation, and acquiring measurement data of the intensities of fluorescence; acquiring a difference between fluorescence intensities and the measurement data, the fluorescence intensities being obtained based on a mathematical model using the diffusion coefficient and the absorption coefficient of the subject itself, the concentration distribution of the phosphor and intensities of the excitation light; updating the concentration distribution of the phosphor so as to reduce the difference; and generating optical tomographic information, from which an optical tomographic image of the subject that is the target of observation that includes the concentration distribution of the phosphor is obtained, from the concentration distribution of the phosphor of which the difference is within a predetermined range.