An image-processing method for processing a time lapse image includes acquiring a first image obtained by capturing an image of an object located within a field of view using an imaging device and a second image obtained by capturing an image of the object located within the field of view using the imaging device after a predetermined time period has elapsed, calculating a positional correction value for a positional shift between the first image and the second image, by using a correlation function weighted according to an image feature of each of the objects included in the first image and the second image, and performing, based on the calculated positional correction value, a positional correction between the first image and the second image. The time lapse image is generated using the first image and the second image on which the positional correction has been performed.