Measurement data of intensities of fluorescence obtained by directing excitation light onto a subject is acquired. An initial value of an absorption coefficient of the phosphor is set on the basis of a concentration distribution of the phosphor, an intensity distribution of the fluorescence on the basis of an absorption coefficient and a diffusion coefficient (reduced scattering coefficient) of the subject, which are set beforehand, are calculated, and the measurement data is compared with the calculation result. If these are found not to be matched, an absorption coefficient of the phosphor at which the error will be a minimum is estimated by performing an inverse problem calculation using a mathematical model. The calculation of the intensity distribution of the fluorescence and evaluation of the error from the obtained concentration distribution are repeated using the absorption coefficient, and a concentration distribution for which the error is the minimum is acquired.