PURPOSE: To perform accurate detection by comparing profile data based on the level change of reflected light with the data of non-defective part.

CONSTITUTION: Strip shaped colored parts 2 and a defective part 3 are provided in a hard disk 1. An arranged data file has 64 arranged data in the radial direction R and 360 arranged data in the circumferential direction S. The data in the direction S are made to increment from 0 to 359, and the data in the direction R are made to increment from 0 to 63. The profile data Y in the direction R are obtained. The length, wherein the data other than 0 continue, and the number of the data are obtained. When both zeroth and 359th data in the direction S are not zero, it is regarded that one discolored part 2 is used, and the number is decreased by one. Thus the profile data for comparison are arranged. When the number of the continuing parts of the data which are not zero is not equal to the number of the discolored parts 2, it is judged that there is a defect. Thus, the quality can detected accurately.