A system comprising a calibration apparatus connected to an optical measurement apparatus, the calibration apparatus being configured to obtain a plurality of calibration data used when the optical measurement apparatus corrects returned light from an object to be measured, the optical measurement apparatus including: a measurement probe having a rod lens, an illumination fiber configured to irradiate via the rod lens to the object to be measured illumination light including at least light of wavelength to be measured, and a plurality of detection fibers (32, 33, 34) configured to receive via the rod lens the returned light, which is of the illumination light reflected and/or scattered by the object to be measured; and a plurality of detection units (23, 24, 25) configured to detect the returned light respectively received by the plurality of detection fibers (32, 33, 34); the calibration apparatus comprising: an insertion portion into which the measurement probe is inserted; and a reference reflection plate that is arranged at a position away from a distal end of the measurement probe by a predetermined distance in a state in which the measurement probe has been inserted in the insertion portion and that has uniform reflectivity of light in a range of the wavelength to be measured in an irradiation plane of the illumination light, wherein a material forming the reference reflection plate satisfies condition or condition as follows: ls * ≥ 2 ⁢ Lsc, AND g ≤ 0.85 ls * ≈ 2 ⁢ Lsc, AND g > 0.85, wherein Is* represents a scattering mean free path of the material forming the reference reflection plate, g represents an anisotropic parameter in a scattering direction of the reference reflection plate, and Lsc represents a spatial coherence length at a detection position of the reference reflection plate and fulfills equation as follows: Lsc = λ ⁢ R / n + L / πρ, wherein a length in a fiber longitudinal direction of the rod lens is R, a distance from the distal end of the measurement probe to the reference reflection plate is L, a refractive index of the rod lens at a prescribed wavelength λ is n, and a core diameter of the illumination fiber is ρ. a measurement probe having a rod lens, an illumination fiber configured to irradiate via the rod lens to the object to be measured illumination light including at least light of wavelength to be measured, and a plurality of detection fibers (32, 33, 34) configured to receive via the rod lens the returned light, which is of the illumination light reflected and/or scattered by the object to be measured; and a plurality of detection units (23, 24, 25) configured to detect the returned light respectively received by the plurality of detection fibers (32, 33, 34); the calibration apparatus comprising: an insertion portion into which the measurement probe is inserted; and a reference reflection plate that is arranged at a position away from a distal end of the measurement probe by a predetermined distance in a state in which the measurement probe has been inserted in the insertion portion and that has uniform reflectivity of light in a range of the wavelength to be measured in an irradiation plane of the illumination light, wherein a material forming the reference reflection plate satisfies condition or condition as follows: ls * ≥ 2 ⁢ Lsc, AND g ≤ 0.85 ls * ≈ 2 ⁢ Lsc, AND g > 0.85, wherein Is* represents a scattering mean free path of the material forming the reference reflection plate, g represents an anisotropic parameter in a scattering direction of the reference reflection plate, and Lsc represents a spatial coherence length at a detection position of the reference reflection plate and fulfills equation as follows: Lsc = λ ⁢ R / n + L / πρ, wherein a length in a fiber longitudinal direction of the rod lens is R, a distance from the distal end of the measurement probe to the reference reflection plate is L, a refractive index of the rod lens at a prescribed wavelength λ is n, and a core diameter of the illumination fiber is ρ.