1. A trapping area creating method comprising: a division step of dividing an image represented in form of a combination of polygons to which colors are applied, into a plurality of image areas by a straight line passing through vertexes of the polygons; a decision step of deciding a propriety of trapping for each pair of image areas adjacent to one another in at least one direction of predetermined two directions, with respect to the plurality of image areas wherein the image is divided; and a trapping area creating step of creating a band-shaped trapping area extending along a boundary of two image areas constituting a pair of image areas decided as being suitable for a trapping in said decision step.