A plate width control device capable of improving the precision of the width of a material to be rolled is provided. In a rolling system, a plate width control device includes an arithmetic unit calculating an estimated value of a deviation amount of the width of the material to be rolled in the vertical rolling mill, and calculating an estimated value of an expansion amount of the width of the material to be rolled when a head end of the material to be rolled is caught in the horizontal rolling mill, and a control unit controlling a gap amount of the vertical rolling mill such that the deviation amount of the width of the material to be rolled is eliminated, and compensating for the gap amount of the vertical rolling mill when the head end of the material to be rolled is caught in the horizontal rolling mill.