To reduce an image data size as much as possible by adding a little hardwares.

An image is scanned by a scan input device 5, plural threshold levels are set by an image converting device 7 so as to convert the image into a binary image according to each level, a fetched multilevel image and plural converted binary images are stored in an image buffer 8 and by successively selectively displaying these images, the binary image according to the suitable threshold level is selected. Besides, by setting plural subtractive coloring levels, the image is converted into color images according to the respective levels, the scanned color image and the respective subtractive color converted color images are stored and by appropriately and selectively displaying the stored images, the color image according to the suitable subtractive coloring level is selected.