the rotary drill bit 13 will provide a bore hole 1 having a profile in the direction of the tool main axis 8 selective directional drilling or straight drilling can thus be achieved in a simple manner by locking or turning the rotary table 5 and hence the drill string 4 and housing 3 by arranging the point of intersection of the rotational axis 16 with the tool main axis 8 to be near to the rotary drill bit 13 the bore hole widens only slightly when operating in the straight drilling mode and compared with the directional drilling operation mode because of the eccentric movement of the rotary drill bit 13 the amount of bore hole widening corresponds to about twice the value of the axial displacement 27 between the rotational axis 16 and the main tool axis 8 in the area of the rotary drill bit 13 this axial displacement 27 is also referred to as offset the offset 27 can be reduced to zero if an eccentric stabiliser is used instead of the centric stabiliser 14 shown in fig2 the eccentricity of the eccentric stabiliser is arranged to compensate for the offset fig3 shows such a modified embodiment in which the stabiliser 18 adjacent to the rotary drill bit 13 is an eccentric stabiliser such an embodiment can be used.