PURPOSE: To highly accurately detect the defect produced in a belt-like sheet to be detected and easily judge the type of defects.

CONSTITUTION: A light beam is irradiated on a sheet 1 which is traveling scanning at a specified angle in the cross direction of the sheet 1 by a light projection section 10 and receives the diffused light from the sheet 1 by a light receiving section 20. At a data processing section 30, abnormal data which exceed a preset threshold out of the output based on the diffused light received and the abnormal position are temporary memorized in a memory device, and the group of the data which are adjacent to in a given condition based on the abnormal position is detected to recognize the correlation coefficient of the group of the data after transforming the coordinate of the abnormal position so that the inclination of the distribution of the data group in the direction of a main spindle may become 1. In addition, the direction of the main spindle to the traveling direction of the sheet is determined to recognize the periodicity of the position data in the traveling direction of the sheet. Then, the defect produced in the sheet is judged from the correlation coefficient, the direction of the main spindle, and the periodicity.