Abstract: A hollow rotary drilling tool is equipped with a percussion device which comprises a nozzle comprising a cylindrical annular duct followed by a divergent annular duct. The divergent duct opens into flow paths, a central path and a peripheral path, through which drilling mud flows alternately in order to cause reciprocation of a percussion head located in the central path. The peripheral path joins the central path downstream of the percussion head. The external conical surface of the divergent duct is connected to the external cylindrical surface of the cylindrical duct via a radial step, which makes the flow in the peripheral path unstable, so that under normal conditions of flow the drilling mud flows along the central path. Flow along the central path is interrupted by contact of the percussion head with an abutment, which limits its movement in one direction, and which closes the central path, causing flow to switch to the peripheral path.