There is provided a surface inspection device configured to detect the surface state of the wafer, such as a defect in the uppermost layer and a variation of the CD value, using even diffracted light influenced by the baselayer. The surface inspection device is configured so that an illumination section illuminates the surface of a wafer with a first illuminating light at a high incident angle which is sensitive to a variation of the surface state of the wafer and a second illuminating light at a low incident angle which is insensitive thereto, a detection section detects diffracted light caused by the high and low incident angles, respectively, and a computing unit determines the CD value after correcting the influence of the baselayer of the wafer based on the information relating to the diffracted lights due to the high and low incident angles.