Referring to FIG. 2 again, the statistical defect-detecting circuit 8 detects small defects which pass undetectedly in defect-detecting circuit 7 in an area unit 90 (see FIG. 6) but together are still counted as defects if the amount of the small defects in a predetermined area, such as an area equals to 30 scanning lines, exceeds a predetermined value (for example, 10 small defects).