To provide a method and a device for correcting a difference image for detecting a shape change that can keep the view of the output difference image constant by making the difference value of a region with no shape change, the fixed value in the difference image for detecting the shape change.

In the method of obtaining the difference image S (x, y) in order to detect the shape change between an input image In1 (x, y) and an input image In2 (x, y), a reference region regarded as having no shape change is set to each image, and the average values ave1, ave2 of the picture element values of the reference region are obtained. The difference image S (x, y) is obtained as S (x, y)=In1 (x, y)-In2 (x, y)-(ave1-ave2) and displayed.