PURPOSE: To provide a method for determining a time for grinding a drill again without falling back on experience and intuition of an operator by quantitatively grasping the amount of abrasion of the drill.

CONSTITUTION: When a drill is abraded by drilling, a feed mark having a width which is in proportion to the feed per revolution of the drill appears on a margin section. When drilling is continuously carried out, several steps of this mark appear. The amount of abrasion of the margin is calculated by counting the number of the steps of this feed mark. When the amount of abrasion of the margin thus calculated becomes equal to, or exceeds, a predetermined amount of repetitive abrasion, the drill is ground again.