PURPOSE: To drill a through hole with good position accuracy by drilling a hole of small diameter first from one surface of a plate material and next drilling a through hole having a finally required dimension, in a position opposite to this hole of small diameter, from the other surface of the plate material.

CONSTITUTION: A prepared hole 1P of diameter and depth smaller than the finally required is drilled in one surface of a workpiece 3 by a prepared hole drill 1 of short cutting edge length, in order to suppress dispersion of a position to the minimum limit. Next, a hole is drilled by a drill, having a finally required diameter and a cutting edge length larger than thickness of the workpiece 3, in the other surface, opposed to the prepared hole 1P, by turning the workpiece 3 upside down, and a through hole is formed. Bending of the hole is generated in the first process, but a dimensional relation between the drills for the prepared hole 1P and for finish is determined in such a way that the diameter of the drill used in a finish process, is increased larger than this bending displacement 2 , so as to prevent the holes from staggering in a part where both holes meet with each other.