1. An image pick-up apparatus for picking up a plurality of images being chronologically arranged along time-series to provide a digital image, comprising: a means for obtaining information of changes in gray level value between those of said plurality of images thus picked up; and a means for deciding a timing at which at least one of said plurality of images is to be selectively extracted among those picked up images, based upon the results obtained by said changed image information obtaining means for using same in an image processing, later on.