In an image processing apparatus and an image reading apparatus in accordance with the present invention, a density difference detection circuit 9 detects the density differences between input image data 400 and storage data 64 stored in an image memory 6 at an overlapping scanning area read by overlap scanning, a density correction circuit 9 corrects the gray levels of the input image data 400 on the basis of the detected density differences, and a mapping circuit 5 stores correction data 910 in the image memory 6.