First and second measurement operations are performed according to each of a plurality of geometric conditions while keeping the geometric condition. In the first measurement operation, illumination light is radiated from a first light radiating position toward a measurement target position and spectroscopic measurement is performed on reflected light traveling from the measurement target position toward a first light receiving position. In the second measurement operation, illumination light is radiated from a second light radiating position toward a measurement target position and spectroscopic measurement is performed on reflected light traveling from the measurement target position toward a second light receiving position. The two spectroscopic measurement results are averaged. The second light radiating position and the second light receiving position are respectively disposed symmetrical to the first light radiating position and the first light receiving position with respect to a reference axis.