1. An image processing apparatus comprising: a reading unit for reading a document, which has been placed on a contact glass, as read image data; a first area determination unit for deciding a provisional document area by subjecting the read image data, which has been read by said reading unit, to detection processing for detecting a document area included in the read image data based upon at least one of a brightness value and color difference value of each pixel constituting the read image data; and a second area determination unit for deciding a fixed document area by generating binary image data that is based upon at least one among brightness, color difference and an edge detection filter in an area corresponding to the provisional document area of the read image data read by said reading unit, and executing detection processing for detecting a document area, which is included in the read image data, based upon the binary image data.