A spool file modifying device for a printing system which generates a spool file including intermediate files independent of a type of printer, and converts the spool file into print data suitable to a target printer. The spool file modifying device includes a spool file retrieving system that retrieves the spool file stored in a storage, a halftone mode designating system that designates an halftone mode as a processing mode to be used when the spool file including a bitmap is reduced in size. With this mode, a gradation of the original bitmap is retained after reduced. The device further includes a modifying system that modifies the retrieved spool file if the spool file retrieved by the spool file retrieving system includes modification information designating the spool file to be modified. The spool file before modified is replaced with the modified spool file, in the storage.