1. An image defect inspection method comprising: detecting a gray level difference between corresponding portions of two images; changing a threshold value in accordance with the distribution of said detected gray level difference; comparing said detected gray level difference with said threshold value; judging one of said corresponding portions to be defective if said gray level difference is larger than said threshold value; computing a brightness difference between said two images, wherein said threshold value is determined in such a manner that said threshold value increases with said computed brightness difference; detecting a positive- or negative-signed gray level difference between the corresponding portions of said two images; computing a cumulative frequency of said detected signed gray level difference; computing a converted cumulative frequency by converting said cumulative frequency so that said cumulative frequency shows a linear relationship to said signed gray level difference when said signed gray level difference is assumed to have a distribution that obeys a prescribed type of distribution; computing an approximate straight line by approximating said converted cumulative frequency by a straight line; and computing said brightness difference in relation to a signed gray level difference having a prescribed frequency in said approximate straight line.