An image processing apparatus generates a first image by adjusting luminance so as to preserve a signal value on a high-luminance side among signal values acquired by reading a document by a scanner, and detects a position of a region corresponding to the document in the generated first image. The image processing apparatus further generates a second image by performing adjustment so as to increase luminance of a pixel contained in the first image, and crops a document image from the generated second image based on the detected position of the region corresponding to the document.