An image processing apparatus sequentially reduces a document, while changing the reduction factor step-by-step. Next, the image processing apparatus refers to the characters that constitute the document that has been reduced with the respective reduction factors, and specifies a reduction factor at which blank regions surrounded by line portions that express each character do not disappear. When an appropriate reduction factor is specified, the image processing apparatus specifies a resolution of the characters for that reduction factor, and converts the resolution of the document data to that specified resolution. Then, the image processing apparatus performs various processing for the document data whose resolution has been converted. Thus the resolution of document data is converted such that the document is reduced with a reduction factor suitable for computer processing.