By using two probe optical systems for measurement by disposing the probe optical systems with a test object sandwiched therebetween, an optical path length of light transmitted through the test object which is identified locally is calculated using an interference signal thereof. In addition, a geometrical thickness of the same part is calculated by measuring positions of the probe optical systems, whereby two calculated values are obtained. Based on the values and a calculated value for a reference object, a refractive index distribution of the test object is obtained.