To reduce running cost by providing non-slip teeth on the working face of a clamp tooth in a centralizer of a drilling machine and the back face thereof respectively.

When the tooth 34 of the front face A side of a clamp tooth 21 has been worn away, a fixing pin 33 is removed and the clamp tooth 21 is removed once from a wrench arm 23 and thereafter, the clamp tooth 21 is reversely turned over to bring the back face B side tooth 35 to the front face side (working side) and fitted to the wrench arm 23. When the tooth 34 of the front face A side of a clamp tooth 22 has been worn away, a fixing pin 33 is removed and the clamp tooth 22 is removed once from a wrench glove 24 and thereafter, the clamp tooth 22 is reversely turned over to bring the back face B side tooth 35 to the front face side (working side) and fitted to the wrench glove 24. In this way, the usable time for one piece of clamp tooth 21, 22 is doubled and hence, the running cost can be reduced.