This invention suppresses a film from rising due to its curl and reduces any blur of an image generated upon projection by using a mirror image print process upon printing on an OHP film. Only when it is determined that a medium of user's choice is an OHP film, the above process is automatically applied. When a print process on an OHP film is designated, a host computer generates a print job appended with a mirror image print designation command, and transmits that print job to a printer. Upon receiving the print job, if the print job contains the mirror image print designation command, the printer converts an image designated by the print job into a mirror image, and executes a print process while feeding an OHP film.