A processing load when an optical tomographic image including a concentration distribution of a phosphor is obtained may be reduced and an apparatus simplified. A measurement section acquires measurement data of intensities of fluorescence obtained by directing excitation light onto a subject, An image processing section sets an initial value of an absorption coefficient of the phosphor on the basis of a concentration distribution of the phosphor, calculates 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, and compares the measurement data 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 the optical scattering equation with a mathematical optimization technique. 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.
