A device (1, 1A) for guiding a movable element for use in a rolling mill with mandrel wherein said movable element is a driving bar of a mandrel or a mandrel, wherein said device comprises a support structure which defines an advancement direction for said movable element, wherein said device (1, 1A) comprises a first slide and a second slide which are carried by said support structure and are slidable along a transverse direction which is orthogonal to said advancement direction, wherein each of said slides (11, 12) carries two abutment surfaces (51-52, 53-54) for guiding said movable element, each of said slides (11, 12) being movable along said transverse direction between at least a first operating position in which, when reached, said abutment surfaces (51-52, 53-54) are susceptible to coming into contact with a movable element of a first predefined diameter, and at least a second operating position in which, when reached, said abutment surfaces (51-52, 53-54) are susceptible to coming into contact with a movable element of a second predefined diameter, said device (1, 1A) comprising actuating means which move said slides (11, 12) along said transverse direction between said operating positions and lock the same when one of said operating positions is reached; characterized in that at least one of said slides comprises a movable portion carrying a first abutment surface of said abutment surfaces (51, 52, 53, 54), in that said movable portion is configured to be movable between a closed position and an open position; and in that in said closed position said first abutment surface is susceptible to contacting said mandrel and wherein in said open position said first abutment surface occupies a position which is distal from said advancement direction and such as to allow the positioning of said mandrel in a housing space delimited by said other abutment surfaces (52, 53, 54). wherein said movable element is a driving bar of a mandrel or a mandrel, wherein said device comprises a support structure which defines an advancement direction for said movable element, wherein said device (1, 1A) comprises a first slide and a second slide which are carried by said support structure and are slidable along a transverse direction which is orthogonal to said advancement direction, wherein each of said slides (11, 12) carries two abutment surfaces (51-52, 53-54) for guiding said movable element, each of said slides (11, 12) being movable along said transverse direction between at least a first operating position in which, when reached, said abutment surfaces (51-52, 53-54) are susceptible to coming into contact with a movable element of a first predefined diameter, and at least a second operating position in which, when reached, said abutment surfaces (51-52, 53-54) are susceptible to coming into contact with a movable element of a second predefined diameter, said device (1, 1A) comprising actuating means which move said slides (11, 12) along said transverse direction between said operating positions and lock the same when one of said operating positions is reached; characterized in that at least one of said slides comprises a movable portion carrying a first abutment surface of said abutment surfaces (51, 52, 53, 54), in that said movable portion is configured to be movable between a closed position and an open position; and in that in said closed position said first abutment surface is susceptible to contacting said mandrel and wherein in said open position said first abutment surface occupies a position which is distal from said advancement direction and such as to allow the positioning of said mandrel in a housing space delimited by said other abutment surfaces (52, 53, 54).