An object of this invention is to provide an image reading apparatus which can reselect a film type after prescanning, and after confirming the result on the monitor, perform actual scanning. To achieve this object, a film scanner selects from a color table storage a color table corresponding to the type of film, sets the color table in an image data converter, and performs prescanning. If the user reselects a film type after prescanning, the color table set before prescanning and a color table selected after prescanning are transferred to a confirmation image changing unit where the difference between the two tables is calculated. Pres-canned image data is transferred from a memory to the confirmation image changing unit, added to the difference, and returned to the memory. The color table selected after prescanning is set in the image data converter, and scanning for obtaining a high-resolution image is executed.