In a method for preparing focus-adjustment data for a focusing lens system of an optical defect-inspection apparatus, a wafer having a plurality of defects is positioned in place with respect to a focal plane defined by the focusing lens system at a positioning step, and the detects on the wafer are optically and electronically detected at a detecting step. Then, defects having a predetermined size are extracted among the detected defects at extracting step, and a number of the extracted defects is counted as defect-number data. The positioning, detecting, extracting and counting steps are repeated whenever the focus-adjustment wafer is relatively shifted from the focal plane by a predetermined distance, and a defect-number distribution is produced based on the defect-number data thus obtained.