PURPOSE: To prevent the break accident of a wire, and also to enhance working efficiency by continuously detecting the amount of wear of the wire in such a state that the wire is traveled.

CONSTITUTION: A pair of conductive rollers 18 are arranged between two points of a wire 14 having an electric resistance consisting of only the wire 14 out of the wire-traveling route, and a current is passed between the two points of the wire 14 through the conductive rollers from a power supply device, and the current value between the two points is continuously measured by an ammeter 44 from the time when there is no wear in the wire 14, and the degree of wear of the wire 14 is detected by the reduction in the current value. In other words, the sectional area of the wire 14 whose diameter has been reduced by wear becomes less than that without wear, and therefore the electric resistance of the former becomes larger. Thus, by utilizing the electric characteristic that the electric current value of the weared wire 14 as compared with that without wear decreases in accordance with the amount of wear, the degree of wear of the wire is detected.