The present invention is characterized in that: a demanded printing-job is accepted at a printing-job accepting unit, and a printing executing unit executes printing according to the accepted printing-job so as to produce an output sheet onto a discharge tray; an output-sheet detecting unit detects the presence of a printed output-sheet on the discharge tray; if the removal of discharged sheets from the discharge tray is detected, a printed-sheet mixture determining unit determines the possibility that sheets printed corresponding to a plurality of demands are mixed among the removed output-sheets; and if it is determined the possibility that sheets printed corresponding to a plurality of the demands are mixed among the output-sheets, warning information is produced by a warning voice producing unit.