PURPOSE: To prevent a drill from escaping and improve drilling efficiency, by using a retaining plate, which forms its bottom in a flat surface while provides in the opposite a required number of surfaces crossing at a right angle with a diagonal hole having a tilt angle, and fixing the flat bottom surface to a flat plate while drilling the diagonal hole with the orthogonal surfaces serving as a guide.

CONSTITUTION: When a flat plate 1 drills many diagonal holes 2, the method uses a retaining plate 4 which forms a flat surface in the bottom while provides in the opposite side a required number of guide surfaces 5 crossing at a right angle with a diagonal hole 2 having a tilt angle. Here the method, fixing the retaining plate 4 to the flat plate 1, drills the guide surface 5 and the flat late 1 to be penetrated by a drill 3 with the guide surface 5 of the retaining plate 4 serving as a guide. Accordingly, the method, enabling improper positioning of the diagonal hole 2 and/or damage of the drill 3 due to its escape to be prevented, can reduce a drilling cost. While the method, when it drills the diagonal hole 2 in the plural flat plates 1, fixed the retaining plate 4 to the flat plate 1, and the method may also easily drill the diagonal hole 2 with a hole, already drilled in the guide surface 5, serving as the guide.