A mandrel holder head with release/fastening device of the mandrel for rolling metallic tubes comprises a mandrel holder (1) which accommodates within an internal seat a half collar (2) of externally cylindrical shape through an opening in the side wall. In the internal seat are accommodated both an end part (52) of the mandrel shank and the half collar (2) through the opening. The half collar has an internal cavity with a second longitudinal side opening which accommodates a second part (51) of the shank of diameter smaller than that of the first part (52). The half collar is divided along an axial direction into two cylindrical parts (21, 22) which have different diameters. The locking member (30, 31) of the half collar (2) holds it in the mandrel holder (1) seat in working conditions, while allowing shank release with an axial movement in locked conditions.