In a device for detecting a paper splice part of a cardboard sheet and a device for producing a cardboard sheet, the device for detecting a paper splice part of a cardboard sheet, which detects a sheet paper splice part of a cardboard sheet that has a first sheet, a corrugated second sheet, and a third sheet bonded together, comprises: a sheet paper splice detection unit that detects the sheet paper splice part on the basis of the shape of the sheet and that is placed between a position at which sheets are spliced and a position at which the sheets are bonded in a sheet carrying direction; a marking device that marks the sheet paper splice part on the downstream side of the sheet paper splice detection unit in the sheet carrying direction; and a mark detection device that detects the mark on the downstream side of the marking device in the sheet carrying direction.