1. An image processing device for recognizing an outline of a moving target comprising: an image pick-up means for taking a plurality of images in times series, in each of which said moving target exists; an image conversion means for converting said images into outline images; a memory for storing said outline images; a first operation means for performing an operation with use of a reference outline image that is one of said outline images corresponding to said image taken at a reference time, a first outline image that is one of said outline images corresponding to said image taken at a first time other than the reference time, and a second outline image that is one of said outline images corresponding to said image taken at a second time other than the reference time and the first time, to provide a processed outline image having only outlines with any positional change that occurs between the reference time, the first time and the second time; a second operation means for performing an operation with use of said reference outline image, and a third outline image that is one of said outline images corresponding to said image taken at a third time other than the reference time, the first time and the second time, to provide a processed outline image having only outlines with any positional change that occurs between the reference time and the third time; and an outline extracting means for extracting the outline of said moving target from the processed outline images provided from said first operation means and said second operation means.