Device for adjusting the path of a rotary drilling tool

A device for adjusting an azimuthal direction of a trajectory of a rotary-drilling bit fastened to an end of a drill string in rotation around an axis thereof arranged substantially along a vertical drilling plane during a drilling of a hole, the azimuthal direction of the trajectory being defined by an angular position of the vertical drilling plane with respect to a reference direction, the device comprising at least one drill string element integral with the drill string in rotation and fastened to the drill string in a vicinity of the drilling bit, the drill string element comprising parts having outer bearing surfaces arranged in a cylindrical surface having an axis merged or substantially merged with an axis of the drill string and a maximum diameter equal to or substantially equal to a nominal diameter of the hole, distributed around the axis of the drill string so as to generate, during rotation of the drill string, by reaction of a wall of the hole on the at least one drill string element, forces tipping the drilling bit out of the drilling plane, either to the right or to the left of the vertical drilling plane, during part of the full rotation of the drill string and to maintain the drill string substantially along an axis of the hole during a remainder of the rotation, and wherein said at least one drill string element comprises at least three radial salient parts directed toward an outside and arranged substantially in 90.degree. with respect to one another around the axis of the drill string, one of the salient parts comprising an outer bearing surface of a size substantially smaller than the size of outer bearing surfaces of the other two salient parts, said bearing part having the small-sized outer bearing surface being placed in a position substantially diametrically opposite with respect to one of the bearing parts comprising a large-sized outer bearing surface so that the drill string element comprises a bearing part on one side of an axial tipping plane passing said bearing part having a small size outer bearing surface.