1. An image processing apparatus comprising: (a) an image reading section for reading an image formed on a document; (b) an image processing section for distinguishing and recognizing an area where the document is present from an area where the document is absent with reference to an area recognition threshold value on the basis of an output of the image reading section; (c) an erasing device for erasing an output image for an area on a recording material corresponding to the area where the document is absent, wherein the erasing device has a rectangle erasing function in which an area inside a minimum quadrilateral including the document is considered to be a document area, and an oblique angle erasing function in which the document area is formed by both edges of each primary scanning line of the document read by the image reading section; and (d) a manual selector for manually selecting any one of the rectangle erasure function, the oblique angle erasure function, or the automatic selection of both of the rectangle erasure function and the oblique angle erasure function.