On receiving FAX image data, an image forming apparatus determines whether or not a security code is superimposed on the image data. If superimposed, the image forming apparatus stored the content of the security code and erases the superimposed security code from the image. Thereafter, the image forming apparatus creates a higher-resolution security code including the same information as that of the stored security code. The FAX image is converted into a higher-resolution image. The higher-resolution security code is superimposed on the FAX image with a higher resolution resulted from the conversion, and the resultant FAX image is printed.