A method of printing an indication of defective printing can print a defective printing mark indicating that printing was not completed normally on a printout containing unprinted or missing dots. When printing one page of print data on recording paper is completed but before a paper discharge command indicating a page break (step ST3) is executed, the operation printing the one page of print data is interrupted and whether or not ink droplets are being discharged normally from the ink nozzles of the inkjet head is detected. If the ink droplets are not being discharged normally, the recording paper is conveyed in reverse direction and print data for the defective printing mark is printed on the recording paper as part of the interrupt process. Therefore, if the recording paper contains unprinted (missing) dots, a defective printing mark is printed on the recording paper and printing the one page of print data then ends.