In an inner layer measurement method, first irradiation light and second irradiation light having a peak wavelength longer than that of the first irradiation light are formed by changing at least one of a position where light emitted from a lamp is transmitted through a short pass filter and a position where light emitted from a lamp is transmitted through a long pass filter. Then, a first XY sectional surface of a semitransparent body is measured by irradiating the first XY sectional surface with the first irradiation light. A second XY sectional surface positioned on a layer deeper than the first XY sectional surface is measured by irradiating the second XY sectional surface with the second irradiation light. Each of the short pass filter and the long pass filter can transmit the light and has properties of changing a cutoff wavelength according to the position where the light is transmitted.