PURPOSE: To enable accurate tracking of the tip of a band plate by determining error rate of feeding of a band plate using a pulse transmitter and a band plate tip detector and correcting pulse period of a pulse signal at the time of tracking.

CONSTITUTION: A pinch roll 3 provided with a pulse transmitter 4 and a wrapper roll 5 are installed at a specified distance. Band plates 2 of various thickness are passed, and the length of the band plates 2 is measured at the time when the tip of the band plate is detected by a detector 7 after arrival at the wrapper roll 5 from the pinch roll 3, and each error rate is determined. The thickness of the band plate 2 being a working condition of a down coiler 1, is inputted to a corrector 8 of a controlling circuit of the wrapper roll 5, and the error rate corresponding to the plate thickness is selected by the corrector 8 and inputted to a pulse frequency dividing circuit 9. A pulse signal (a) is inputted to the pulse frequency dividing circuit 9 from the pulse transmitter 4. The pulse period a' is corrected to period b' by the error rate and outputted as a pulse signal (b). Thus, the accurate tracking of the tip of the band plate is performed.