A paper jam detection system for a folding machine includes an interval signal generator for generating an interval signal which assumes ON and OFF levels alternately according to cutting intervals of a cutting cylinder; a signature detection mechanism including reflection plates each having a light reflection surface and being arranged circumferentially at an outer circumferential surface portion of a jaw cylinder and a photoelectric sensor adapted to generate a reflection plate detection signal upon detection of the reflection plate; and a control unit for outputting a paper jam signal on the basis of the interval signal and the reflection plate detection signal.