PURPOSE: To give a correction value to a NC device for high-precision perforation by finding the shifts of a main shaft due to thermal displacement with the movement of the main shaft in accordance with the positional information of a displacement sensor from the NC device with respect to the initially-set alignment of the main shaft.

CONSTITUTION: A relative displacement amount between the position of a reference bush hole 6 in a sub-table 5 and the position of a main shaft 4 is read on a dial gauge to set displacement data in an X-axial direction as x2 and displacement data in a Y-axial direction as y2. In accordance with positional information given from a NC device, the main shaft 4 is moved to measure displacement data ax, ay from the center of a displacement sensor 7 for the main shaft 4, and the alignment correction values of the main shaft 4, X1=x1-ax, Y2=y2-ay, are found. Positional data in the NC device is rewritten in accordance with the correction values and the initial setting for alignment of the main shaft 4 is performed. When perforation starts, thermal displacement is caused by heat due to high speed rotation. Prior to starting the next machining cycle, the main shaft 4 is moved to the displacement sensor 7 to detect the shifts bx, by and the correction values X=x1-bx, Y=y1-by are found.