PURPOSE: To display out image data with an optimum magnification for reading characters by recognizing the size of main characters contained in the image data and calculating the magnification corresponding to that character size and the characteristics of the display device.

CONSTITUTION: When a central processing unit (CPU) 10 performs control to read the image data from an external storage device part 14, character data are segmented from the read image data by a character segmenting part 15. The character size (dot) of the segmented character data by a character size measuring instrument part 16. Then, the segmentation of characters and the measurement of the character size are repeated suitable times, the most frequently apearing character size is defined as a reference character size, and the magnification of images is calculated by a magnification calculating device part 17 so that the characters can be displayed on the display device at a readable degree (such as from 12 to 16 dots in the case of Chinese characters, for example). Then, the image data are enlarged/reduced according to the calculated magnification and displayed on the display device.