Abstract: Such defects as a folded back or bent portion and a notch along one or both side edges of a printing sheet is detected by a detector located on one or both sides of a front edge of a correctly fed printing sheet for detecting absence of the sheet and a timing signal generator which produces a timing signal while all length of the sheet passes by the detector. An output signal which stops printing operation is produced when the detector detects absence of the sheet while the timing signal is being produced. According to this invention it is possible to prevent formation of defective printed matter and damage of a printing press fed with defective printing sheets.