PURPOSE: To prevent paper from being torn by storing and holding output data of the reference speed of computer in a normal state for a back device for the computer for a paper machine, etc., even when the computer is abnormal and by performing manual operation.

CONSTITUTION: Backup device 7, while outputting interruption signal (k) for a diagnosis of a computer to CPU1, receives analog reference signal (b) and performs A/D conversion and, when diagnostic response signal (l) arrives, compares it with a value stored at the last timing. As a result, when it is equal to the last memory value, this value is D/A-converted and outputted as automatic speed reference signal (m) to paper machine controller 5. When they disagree, on the other hand, the current A/D-conversion data is held and signal (m) is left as it was. Further, if diagnostic interruption response (l) is not made to diagnostic interruption (k), signal (m) stays as it was. Consequently, even if the computer is abnormal, output data of the reference speed when the computer is normal is recorded and held and manual operation is started, so that the tearing of paper is prevented.