1. A method for generating optical tomographic information comprising: setting beforehand a diffusion coefficient and an absorption coefficient of a subject with respect to excitation light that, by being illuminated at a phosphor inside the subject, is capable of causing luminescence, and an absorption coefficient of the phosphor according to a concentration distribution of the phosphor; illuminating the excitation light from a light source toward a surface of the subject; detecting, at a periphery of the subject, intensities of fluorescence that is caused to be emitted from the phosphor by the excitation light illuminated at the subject and is emitted from the subject, and acquiring measurement data; acquiring a difference between fluorescence intensities and the measurement data, the fluorescence intensities being obtained from a mathematical model based on the diffusion coefficient and the absorption coefficient of the subject and the absorption coefficient according to the phosphor concentration distribution that have been set beforehand and the excitation light emitted from the light source; updating the absorption coefficient of the phosphor so as to reduce the difference and repeating the acquiring of the difference; and on the basis of a concentration distribution of the phosphor acquired from the absorption coefficient of the phosphor, generating optical tomographic information that forms an optical tomographic image of the subject.