A microwave sending antenna 3R and a microwave receiving antenna 4R are provided to a right-side furnace wall 1R, and the system is devised so that the microwaves emitted from the microwave sending antenna 3R are reflected by the right-side edge of a cold-rolled steel plate 2, and the reflected waves are received by the microwave receiving antenna 4R. Similarly, a microwave sending antenna 3L and a microwave receiving antenna 4L are provided to a left-side furnace wall 1L, and the system is devised so that the microwaves emitted from the microwave sending antenna 3L are reflected by the left-side edge of the cold-rolled steel plate 2, and the reflected waves are received by the microwave receiving antenna 4L. If the time from the sending of the microwaves until the receiving of the reflected waves is designated as t, and the velocity of the microwaves is designated as c, then the distance to the object reflecting the microwaves is determined from t·c/2. It is possible to provide a method for measuring the width-direction end position of a material strip with which noise and atmosphere have little effect, installation is easy, and there is little mechanical interference with the material strip.