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 formed by RGB color components; 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 first detection processing for detecting a document area included in the read image data based upon a brightness value of each pixel constituting the read image data; and a second area determination unit for deciding a fixed document area based on both an area defined by binary image data and the provisional document area, wherein said second area determination unit determines whether the following i), ii) and iii) requirements are satisfied for each pixel of the read image, and generates the binary image data by setting a value for each pixel to one if at least one of the following i), ii) and iii) requirements is satisfied and to zero otherwise: i) the brightness value of grayscale image data which is converted from the read image data does not exceed a given threshold, ii) the difference between the maximum and minimum RGB color components of each pixel constituting the read image data is not less than a given threshold, and iii) the pixel of the grayscale image data which is converted from the read image data is part of an edge which is determined by using an edge detection filter.