A first document image is scanned from a document in a first orientation, and a second document image is scanned from the document in a second orientation different from the first orientation. A wrinkle area candidate detecting unit rotates the first document image by a rotation angle between the first and second orientations, and detects as a wrinkle area candidate an object in a predetermined density range in the rotated first document image and the second document image. A similarity degree determining unit determines a similarity degree between the wrinkle area candidate of the rotated first document image and the corresponding wrinkle area candidate of the second document image. A wrinkle area removing unit removes the wrinkle area candidate as a wrinkle area if the similarity degree falls into a predetermined range.