Abstract: Apparatus is provided for supporting the mandrel back-up bar of a press-piercing rolling mill. The apparatus comprises a plurality of constraining elements supported by a base in two lines disposed parallel to and symmetrically with respect to the rolling and piercing axis of the rolling mill. All the constraining elements of each line are mounted on a single beam which is supported on horizontal guides perpendicular to the rolling axis and adjustable in position on the guides. This arrangement facilitates the simultaneous adjustment of all the constraining elements making up a line.