PURPOSE: To improve boring process of steel by setting a core thickness of a twist drill as 25 to 40% of a knife edge diameter, a groove width ratio is 0.8 to 1.0:1, and a rake angle in a radial direction as -6 to -10 degrees.

CONSTITUTION: A rake angle theta at a knife edge 4 of a drill body 2 is set as -6 to -10 degrees referenced to a peripheral part, for the improvement of defect and in sharpness. A ratio of the width of a land part 8 and that of a groove part 5 is set within a range of 0.8 to 1, for the compatibility of exhaust of a chip and in maintenance of rigidity. Also, the core thickness is designated as 25 to 40% of an edge diameter, for the improvement of both rigidity and exhausting characteristic. In making the structure such as this, deposition of a chip to a chisel edge 11 is reduced and tenacity is also improved.