In an image defect inspection method and apparatus which detects a gray level difference between corresponding portions of two images, automatically sets a threshold value based on the distribution thereof, compares the gray level difference with the threshold value, and judges one or the other of the portions to be defective if the gray level difference is larger than the threshold value, provisions are made to correct the threshold value when the distribution of the gray level difference is different from the usual distribution, thereby achieving high detection sensitivity while suppressing the occurrence of false defects. To achieve this object, the cumulative frequency of the gray level difference is computed (S3); a converted cumulative frequency is computed by converting the cumulative frequency so that the cumulative frequency shows a linear relationship with respect to the gray level difference when the gray level difference is assumed to exhibit a distribution that obeys a prescribed type of distribution (S4); an approximation curve of the converted cumulative frequency is derived (S11); the second derivative of the approximation curve with respect to the gray level difference is computed (S12); and the threshold value is corrected in accordance with the second derivative (S13).
