A rock anchor bolt, including - a drawbar with an inner end to be inserted into a borehole and an outer end extending out from the borehole and having an external thread, - a support plate for supporting an outer end of the rock anchor bolt against the surface of the rock around the mouth of the borehole, the support plate having a center hole, - a centering member provided at the outer end of the drawbar in the center hole of the support plate, the centering member including a central hole through which the outer end of the drawbar is adapted to extend and an injection hole through which grouting paste can be injected, - a tightening nut turned on the external thread of the outer end of the drawbar against the centering member in order to draw the drawbar outward relative to the borehole for tightening the anchor bolt in its place, - a conical member connected substantially immovably to the drawbar in the vicinity of the inner end of the drawbar, - an expansion member inside which the conical member is provided, the expansion member being adapted to expand as the drawbar is being drawn outward by means of the tightening nut and the conical member is moving at the same time relative to the expansion member for expanding the expansion member and making it adhere to the wall of the borehole, - a tube made of a rigid material and fitted around the drawbar so that a casting space is provided between the tube and the drawbar for receiving and carrying grouting paste injected through the injection hole of the centering member, the tube including an outer end supported on the centering member, an inner end and casting holes (19, 20) in order to allow the injected grouting paste to flow between the space between the tube and the wall of the borehole and the casting space, characterized in that the centering member includes an outlet hole for removing any leakage water flowing into the borehole; that the rock anchor bolt includes a sealing device which is provided around the outer end of the tube to seal the borehole watertight as the tightening nut is being tightened, and an outlet tube which is provided in the casting space between the tube and the drawbar to carry out the leakage water as the grouting paste is being injected, the inner end of which outlet tube extending to the vicinity of the expansion member, the outer end of the outlet tube extending out through the outlet hole of the centering member; and that the tube has one or more first casting holes at the outer end of the tube in the vicinity of the sealing device and one or more second casting holes at the inner end of the tube in the vicinity of the expansion member, whereby, as grouting paste is being injected, it fills the casting space between the tube and the drawbar as well as the space between the borehole and the tube from an outer end toward an inner end so that any leakage water is forced ahead of the grouting paste to the inner end of the outlet tube. - a drawbar with an inner end to be inserted into a borehole and an outer end extending out from the borehole and having an external thread, - a support plate for supporting an outer end of the rock anchor bolt against the surface of the rock around the mouth of the borehole, the support plate having a center hole, - a centering member provided at the outer end of the drawbar in the center hole of the support plate, the centering member including a central hole through which the outer end of the drawbar is adapted to extend and an injection hole through which grouting paste can be injected, - a tightening nut turned on the external thread of the outer end of the drawbar against the centering member in order to draw the drawbar outward relative to the borehole for tightening the anchor bolt in its place, - a conical member connected substantially immovably to the drawbar in the vicinity of the inner end of the drawbar, - an expansion member inside which the conical member is provided, the expansion member being adapted to expand as the drawbar is being drawn outward by means of the tightening nut and the conical member is moving at the same time relative to the expansion member for expanding the expansion member and making it adhere to the wall of the borehole, - a tube made of a rigid material and fitted around the drawbar so that a casting space is provided between the tube and the drawbar for receiving and carrying grouting paste injected through the injection hole of the centering member, the tube including an outer end supported on the centering member, an inner end and casting holes (19, 20) in order to allow the injected grouting paste to flow between the space between the tube and the wall of the borehole and the casting space, characterized in that the centering member includes an outlet hole for removing any leakage water flowing into the borehole; that the rock anchor bolt includes a sealing device which is provided around the outer end of the tube to seal the borehole watertight as the tightening nut is being tightened, and an outlet tube which is provided in the casting space between the tube and the drawbar to carry out the leakage water as the grouting paste is being injected, the inner end of which outlet tube extending to the vicinity of the expansion member, the outer end of the outlet tube extending out through the outlet hole of the centering member; and that the tube has one or more first casting holes at the outer end of the tube in the vicinity of the sealing device and one or more second casting holes at the inner end of the tube in the vicinity of the expansion member,