PURPOSE: To reduce the misdetection of a defect and to improve the accuracy of defect inspection by two-dimensionally arraying flaw detection signals with relation to the scanning positions of a probe, detecting the directional property of a defect in the two-dimensional flaw detection signal obtained by the array and extracting the surface defect.

CONSTITUTION: The scanning of the probe C is started from one side end of a material E to be inspected in its width direction and executed by combining main scanning parallel with the width direction and ending its operation at the other side end with subscanning for moving the positions of the main scanning in parallel with the end part of the material E in the longitudinal direction successively in each fixed interval. Flaw detection signals obtained by surface defect inspection are two-dimensionally arrayed with relation to the scanning positions of the probe C. Prescribed processing is applied to a two-dimensional flaw detection signal obtained by the array to discriminate the directional property of a defect signal and the defect signal having directional property required for inspection is extracted. When a standard deviation is set up as the scale of required defect detection, a fine surface defect can be also highly accurately detected.