An inner layer measurement method of measuring XY sectional surfaces of a semitransparent body by means of interferometric microscopy, in which light is reflected on a surface and an inner layer thereof, the method comprising: emitting light having both wavelength bands of first irradiation light and second irradiation light which has a peak wavelength longer than that of the first irradiation light; characterized in that the method comprises forming the first irradiation light and the second irradiation light by transmitting the emitted light through a short pass filter and a long pass filter and changing at least one of a position at which the light is transmitted through a short pass filter and a position at which the light is transmitted through a long pass filter according to a measurement position of the semitransparent body in a Z axis direction; measuring a first XY sectional surface of the semitransparent body by irradiating the first XY sectional surface with the first irradiation light; moving the measurement position of the semitransparent body in a Z axis direction and thus changing a depth of the measurement position from the surface of the semitransparent body, and measuring a second XY sectional surface positioned on a layer deeper in the Z axis direction than the first XY sectional surface by irradiating the second XY sectional surface with the second irradiation light, wherein the short pass filter is capable of transmitting the light and has properties of changing a cutoff wavelength in response to the position at which the light is transmitted, and the long pass filter is capable of transmitting the light and has properties of changing a cutoff wavelength in response to the position at which the light is transmitted, the wavelength band of first irradiation light is determined according to the depth of the measurement position for the first XY sectional surface and the wavelength band of second irradiation light is determined according to the depth of the measurement position for the second XY sectional surface. emitting light having both wavelength bands of first irradiation light and second irradiation light which has a peak wavelength longer than that of the first irradiation light; characterized in that the method comprises forming the first irradiation light and the second irradiation light by transmitting the emitted light through a short pass filter and a long pass filter and changing at least one of a position at which the light is transmitted through a short pass filter and a position at which the light is transmitted through a long pass filter according to a measurement position of the semitransparent body in a Z axis direction; measuring a first XY sectional surface of the semitransparent body by irradiating the first XY sectional surface with the first irradiation light; moving the measurement position of the semitransparent body in a Z axis direction and thus changing a depth of the measurement position from the surface of the semitransparent body, and measuring a second XY sectional surface positioned on a layer deeper in the Z axis direction than the first XY sectional surface by irradiating the second XY sectional surface with the second irradiation light, wherein the short pass filter is capable of transmitting the light and has properties of changing a cutoff wavelength in response to the position at which the light is transmitted, and the long pass filter is capable of transmitting the light and has properties of changing a cutoff wavelength in response to the position at which the light is transmitted, the wavelength band of first irradiation light is determined according to the depth of the measurement position for the first XY sectional surface and the wavelength band of second irradiation light is determined according to the depth of the measurement position for the second XY sectional surface.