1. An image outline determination method of determining an outline of an original image placed on a reading surface of a scanner in accordance with image data obtained by the scanner, said image outline determination method comprising: an area designation step of designating a rectangular proposed area of which at least portion enters into an area of the original image placed on the reading surface; and an outline determination step of determining the outline of the original image by repeating a plurality of number of times an area alteration processing in which a movement of sides defining an outline of the proposed area outside or inside the proposed area is determined in accordance with differential or difference of image data for vicinity of the sides, and the proposed area is altered in accordance with a decision of the movement, said area alteration processing being repeated taking an area after the proposed area is altered as a new proposed area.