PURPOSE: To dissolve defects that a quantity of supply of coil when a line speed is reduced is change by the steady speed of the line, further, an operation accuracy is bad, a large threshold value for the remainder of the coil must be required and the working efficiency is low due to the long time of the low speed operation.

CONSTITUTION: An arithmetic circuit for the remainder in an uncoiler is constituted by using a digital arithmetic unit of a general purpose sequencer. In monitoring the remainder in the uncoiler, a detected line speed (v) and a number N of rotation of a detected coil are input into this arithmetic unit (S1). The remainder I of the coil and the threshold value lt for the remainder of the coil are calculated (S2, 3) by using the inputted data. The threshold value lt for the remainder of the coil is made a value suitable for the amount of supply in the time of a reduced line speed. When the remainder I of the coil is compared with the threshold value lt for the remainder of the coil (S4) and l≤lt is obtained, an indication for starting a reduced line speed is outputted to the outside (S5).