PURPOSE: To provide a dril tip which is periodically ground and compensated for the tool length, and a tool length control device.

CONSTITUTION: A drill tip grinding device K is arranged on a corner of a table 2, and when a drill, drilling operation of which is interrupted periodically or based on prediction information from a cutting operation monitoring device, is ground by the drill tip grinding device K, grinding of the drill D and compensation of the tool length L are performed by renewing the information on the position of drill tip after the drill is ground by a specified amount by means of a NC control device.