1. An image defect inspection method which detects a gray level difference between corresponding portions of two images, compares said detected gray level difference with a threshold value, and judges one or the other of said portions to be defective if said gray level difference is larger than said threshold value, comprising: a cumulative frequency computing step for computing a cumulative frequency of said gray level difference between said two images; a converted cumulative frequency computing step for computing a converted cumulative frequency by converting said cumulative frequency so that said cumulative frequency shows a linear relationship with respect to said gray level difference when said gray level difference is assumed to exhibit a distribution that obeys a prescribed type of distribution; an approximation curve deriving step for deriving an approximation curve of said converted cumulative frequency; and a threshold value correcting step for correcting said threshold value in accordance with a second derivative of said approximation curve with respect to said gray level difference, wherein said comparison is performed by using said corrected threshold value.