A calibration for a scanner, which is used as a densitometer, is carried out always with high accuracy. More specifically, an image processing method of generating a conversion condition for a scanner which is used for reading an image and generating image data, is provided. The method including the steps of: obtaining a reading property of an object scanner based on image data obtained by that the object scanner reads a chart, and generating the conversion condition for the object scanner based on the reading property of the object scanner, a previously prepared reading property of a standard scanner and a previously prepared brightness-density conversion condition for the standard scanner.