PURPOSE: To form a deep hole with the exceedingly high precision and straightness, by installing a bearing guide pad which receives the main cutting force on the side surface of a tool head and installing a guide part which receives the back component force on the cutting edge on the side for receiving the back component force.

CONSTITUTION: On the side surface of a twist dril head 10, a bearing guide pad 13 is installed in the loading direction of a main cutting force P1, and at the edge of a cutter 12 in the loading direction of the back component force P2, a guide part 14 which is formed to an R shape is formed. Accordingly, the main cutting force P1 is received by the bearing guide pad 13 on the side surface of the twist drill head 10. Further, the force P2 of component is received by the guide part 14 formed to the R shape at the edge of the cutter 12. The back component force generated during the cutting by the development of the dimension determining guide of the guide part 14 and the guide part 14 which receives this pressure and possesses the dimension determining action are balanced in pressure aspect correctly, and a deep hole having the exceedingly high precision and straightness can be formed.