Adjustment device and method to guide a metal product (11) along a determinate nominal rolling axis (M). The device comprises a central body (13) disposed astride the rolling axis (M), two guide units (15, 16) mounted movable on the central body (13), opposite each other with respect to the rolling axis (M) and each provided with at least a guide roller (20) for the metal product (11), and an adjustment mechanism (17) mounted on the central body (13) and operatively connected to both the guide units (15, 16), in order to actuate the selective movement thereof with respect to the central body (13) at least in an approach toward the rolling axis (M). The adjustment mechanism (17) comprises a system of pantograph levers having two levers (21, 22) pivoted to each other and to the central body (13) in correspondence to the rolling axis (M) by means of a pin (23), and first constraint means (25) disposed on each lever (21, 22) on opposite sides to the pin (23), so as to connect each lever (21, 22) to both the guide units (15, 16).