An image processing apparatus including, an input unit for inputting image data including a document region, a storage for storing the brightness of each pixel on a first main scan line outside the document region, a corrector for correcting the brightness of each pixel on the first main scan line in accordance with the brightness of pixels in a first part and the brightness of pixels in a second part on a second main scan line following the first main scan line, and an edge point detector for detecting an edge point of the document region situated on the second main scan line in accordance with the difference between the brightness of each pixel on the first main scan line corrected and the brightness of each pixel on the second main scan line respectively corresponding to each pixel on the first main scan line.