Abstract: An optimum printing mode, which makes total printing through-put maximum, is selected by providing control programs comprising monitoring means 4 for monitoring printing condition of the printer, comparative means 5 for determining the printing mode which makes the printing time minimum by comparing the printing mode at the moment of the printer 30 obtained by the monitoring means 4 with the printing mode of the subsequent data to be printed based on information relating to a necessary time for switching the printing mode, difference of printing time necessary for one page with respective of the printing modes each other, and the number of pages to be printed, and others, switching means 6 for switching whether the optimization of the printing mode determined by the monitoring means 4 and the comparative means 45 is performed or not and selective means 7 for selecting the printing mode based on the result determined by the comparative means 5 or setting by the switching means 6, to printing controlling portio