1. A method of detecting the direction of a document image, comprising the steps of: inputting the document image in original multi-value data; digitizing the multi-value data into digitized image data; detecting the direction of the document image based upon the digitized image data; in case the direction detection failed, reversing brightness of the original multi-value data into brightness reversed image data; digitizing the brightness reversed image data into digitized brightness reversed image data; and detecting the direction of the document image based upon the digitized brightness reversed image data.