To accurately detect the generation of end-blocking deemed undetectable.
A device for detecting a slip is provided with: a detecting means 14 for a driving torque current of the main roll of a piercer, a detecting means 18 for a driving torque current of a disk roll 15, and a master controller judging the generation of end-blocking when the driving torque current of the disk roll 15 increases exceeding a certain value after the detection signals from both detecting means 14 and 18 are input and load relay signals from the driving torque current of the main roll are cut off. Thereby, the generation of end- blocking can be accurately detected and the seizure of the master roll and the disk roll can be previously prevented owing to the detection. Besides, the generation of larger troubles can be previously prevented.
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