A scan reading method according to this invention comprises the steps of: causing a line memory to store read original image data for one line; obtaining a density average value of one-line image data or a variation rate thereof; and detecting the edge of the original by comparing the average density value or the variation rate thereof with a predetermined threshold value after finishing a scan over the original or during the scan. A scan reading method also comprises the steps of: preparing a density histogram from the one-line image data in the original region; optionally obtaining the average density value of the variation rate thereof; detecting the top end and terminal of the original from that value; and calculating highlight and shadow densities from the density histograms after finishing the scan over the original by use of a scanner. Based on the methods of the present invention, it is therefore possible to automatically detect the edge of the original and perform a density auto-measurement at low costs with a high accuracy with the aid of a small capacity line memory without adding a multiplicity of sensors and incorporating a large capacity buffer memory.