A measurement light source is arranged on a rear surface side of a surface at a sample position irradiated with light by an irradiation light source and a detector is arranged on a front surface side of the surface at the sample position irradiated with light by the irradiation light source. A radiation intensity calculator calculates a radiation intensity at each wavelength of irradiation light from the irradiation light source based on a first detected intensity distribution, a second detected intensity distribution, and radiation characteristics of a standard light source. An irradiated photon number calculator calculates the number of irradiated photons at each wavelength of irradiation light from the irradiation light source based on the radiation intensity at each wavelength.