Light from a light source becomes two illumination beams by a beam splitter. The beams are irradiated onto a semiconductor wafer from two mutually substantially orthogonal azimuthal angles having substantially equal elevation angles to form illumination spots. When the sum of scattered, diffracted, and reflected lights due to the illumination beams is detected, influence of the anisotropy which a contaminant particle and a defect existing in the wafer itself or thereon have with respect to an illumination direction, can be eliminated.