A test unit is formed by sandwiching a test lens by a first reference lens whose shape and refractive index are known, a second reference lens whose shape and refractive index are known, and a medium. A wavefront of light transmitted through the test unit is measured, and, by using the shape and the refractive index of the first reference lens, the shape and refractive index of the second reference lens, and the measured wavefront of the test unit, the refractive index distribution of the test lens is calculated.