Abstract: Earth drilling apparatus comprising an extensible mast constituted by a fixed lower ramp and an upper slide mounted with capability of longitudinal sliding on the fixed ramp. A rotation head for driving a drill string is movable on and guided over the length of the slide and the movements of extension and retraction of the slide are synchronized with ascending and descending displacements respectively of the rotation head. For this purpose two double-acting jacks connect the lower portion of the fixed ramp to the upper portion of the movable slide and each of a first pair of chains is wound on one pulley mounted on the upper extremity of each of the posts of the movable slide and is fixed at one of its extremities to the rotation head and at the other of its extremities to a cross-brace rigidly connecting the upper extremity of the posts of the fixed ramp.