PURPOSE: To improve space resolution on measurement by setting an incident spot on a silicon wafer surface to a slender form, and moving the spot in the short axial direction.

CONSTITUTION: An incident spot 3 on the surface of a silicon wafer 1 formed by an incident light beam is an ellipse having a ratio of long axis to short axis of about 3.56, since the angle of polarization of the wafer 1 is set about 73.7°. The interstitial oxygen concentration distribution of the wafer 1 is measured while moving the spot 3 in the short axial direction. The interstitial oxygen concentration distribution of the wafer 1 tends to have a point- symmetrical form resulted from its manufacturing process, and is the same on the circumference with the same radius from the center. Thus, a result with more excellent space resolution can be provided when the concentration distribution is measured by moving the spot 3 in the axial direction.