1. An image-reading apparatus that reads an image of a document comprising a Central Processing Unit (CPU) programmed to: determine any one or both of a type of document and a shape of a document; select any one or both of an appropriate document tilt detection method and an appropriate document position detection method according to any one or both of the determined type of document and the determined shape of the document; detect any one or both of a tilt of the document and a position of the document by using the selected detection method; and when clipping is performed by using a detected rectangle, determine whether an occurring lack of a document exceeds a predetermined acceptable level.