To efficiently process a deep and small diameter hole having a diameter of about 6 mm or less in a highly hard work having hardness of 45 HRC or more by using a drill.

The drill 12 has an end diameter D of 6 mm or less and an axial length L1 of an underhead portion of a body 32 ten times or more as large as the end diameter D and can drill the deep and small diameter hole ten times or more deeper than the end diameter D. In this drill, an axial length L2 of chip discharging grooves 36 is in a range 2 to 5 times larger than the end diameter D to obtain relatively high mechanical strength. A hard coating is coated over an entire area of a surface of the underhead portion of the body 32, and good cutting performance and the like of cutting edges 34 are maintained. This provides practically satisfactory tool life even for the highly hard work having hardness of 45 HRC or more, and processing efficiency is largely improved as compared with electric discharge machining.