To solve the problem that high-accuracy color measurement is difficult although many image can be read at one time in an image reading device such as a flat head scanner.

An object to be read is scanned to acquire read image data including a gradation value for each color component in a plurality of pixels. A scan profile indicating correspondence relation between the gradation value and a color value and defining the correspondence relation for each scanning position of a plurality of scanning positions in the image scanning process, is obtained. The above gradation value is converted into the color value for the read data acquired by the image scanning process referring to the scan profile corresponding to each scanning position.