PURPOSE: To prevent damage to a printing device or failure of printing operation by allowing a sensor extending over the entire sheet width to detect the bulge of a fed sheet, and converting an acquired measurement value into a switching impulse acting on a drive device, for removing a defective carton sheet.

CONSTITUTION: A sensor for detecting the bulge of a sheet 1 comprises blow popes 3 distributed densely, side by side, in lateral direction over the entire width of the sheet 1. The lower end parts of openings 4 of the blow pipes are almost vertical to the surface of the sheet 1 which is transported through a sheet feeder table 2. All the blow pipes 3 are connected to a common measurement chamber 10 provided in a pipe 11 extending over the entire width of the sheet 1 through a branch pipe 9. The measurement chamber is connected to a pressure sensor 12, and the measurement value of the pressure sensor is converted into a switching impulse for cutting off the drive device of sheet feeder or the drive device of a following printing device.