1. An image defect inspection method which detects a gray level difference between corresponding portions of two images, changes a threshold value in accordance with the distribution of said detected gray level difference, compares said detected gray level difference with said threshold value thus determined, and judges one or the other of said portions to be defective if said gray level difference is larger than said threshold value, wherein a brightness difference between said two images is computed, and said threshold value is determined in such a manner that said threshold value increases with said computed brightness difference.