PURPOSE: To enable drilling fully precisely by a single machining process, in making a high precision through hole in a workpiece, by forming a part of the through hole with a diameter larger than the nominal one.

CONSTITUTION: A beam 6 generated by a laser 5 is converted to a first drilling beam having a comparatively large opening angle and a second drilling beam having a comparatively small opening angle, by means of an adjustable lens system 8 arranged between a polarizing mirror 7 and the wall 1 of a workpiece. The focal point P1 of the first drilling beam 9 is below the wall 1 inside the deepest bottom of the blind hole 3 having a diameter larger than the nominal one. The focal point P2 of the second drilling beam 10 is outside the wall 1. As a result, the drilling transverse section 4 having the nominal diameter is provided with a cylindrical shape as much as possible.