PURPOSE: To make the depth of a blind hole correct by measuring the descent quantity until the tip of a drill comes in contact with a blade tip measuring sensor provided at one corner of a workpiece installation base, and setting the descent quantity on the basis of the measured value after a cutting edge quantity monitor sensor provided at a spindle head detects the clearance measure to the upper face of a workpiece.

CONSTITUTION: The descent quantity from a co-ordinate basic origin P until a drill 13 comes in contact with a blade tip measuring sensor 16 provided on an installation base 17 is measured and compared with the distance H to the base surface from the tip of the drill 13 at the time of the drill 13 being normally chucked to a spindle head 12, and its dispersion error is set as the correction value δ at the machining time. The correction value is then added to the reference clearance measure α to the upper face of a workpiece detected by a cutting edge quantity monitor sensor 11 provided at the spindle head 12, and the cutting edge descent quantity (h) of a blind hole is further added thereto. The drill descent quantity at the time of detecting the reference clearance measure α is made K=H+δ+h, and the blind hole can be machined into the correct depth.