To measure accurately a primary component (inclination component) of a refractive index distribution of a test object.

A measuring device for measuring the wave front shape of light transmitted through the test object is used, and the attitude of the test object is changed to a position determined by being rotated around the optical axis of the measuring device as much as a prescribed angle, and first measurement data and second measurement data outputted respectively before and after the change by the measuring device are acquired, and the difference between the first measurement data and the second measurement data is determined as information for showing the primary component (inclination component) of the refractive index distribution of the test object. The primary component (inclination component) of the refractive index distribution follows the attitude change, but a primary error component generated in the measuring device does not follow it. Therefore, the primary component (inclination component) of the refractive index distribution appears in the difference between the two measurement data, but the primary error component in the measuring device does not appear therein.