PURPOSE: To improve those of roundness, cylindricality, surface roughness and dimensional accuracy in a hole, by installing a broad front guide in a shunk tip while a pocket in a tip at the rear, respectively.

CONSTITUTION: Both front and rear guides are made to be broad enough in size. A cutting edge 311 is formed at the front part of a tipe 31, having a negative rake angle α. This cutting edge 311 is tilted as far as a portion for an angle β in relation to an axial center of a reamer. The rake angle α is set to a negative range whereby any damage to a work surface due to a bite of chips will not occur, and all chips are discharged outside along an oil groove. Likewise, with a back taper 312 being formed therein, roundness and cylindricality are all improved ever so better.