To solve the problem wherein, when a perpendicular hole is drilled on the lateral face of a plate material using a portable electric drill, an index to confirm the perpendicular posture of the drill is not provided in the vicinity of a drilling point and it is difficult to accurately mark off the midpoint of the plate width on the lateral face of the plate material.

This auxiliary tool provides an index to make a drill maintain a perpendicular posture by nipping drilling planes by two thin plates with perpendicular reference faces and the midpoint of the plate width. The thin plates are interconnected by two columns to enable parallel displacement, and by making the two columns coming into close contact with the top faces of the drilling planes, the two columns are arranged in positions where the two perpendicular reference faces have perpendicular postures with respect to the drilling planes. On the inner lateral faces of the two thin plates, the two thin plates of congruent right-angled triangles are fixed in slidable positions, respectively so that the slant edge faces intersect with each other and the two thin plates are superposed on each other. When the two thin plates with the perpendicular reference faces nip a material to be drilled and are fixed, the intersection point of the slant edge faces of the two right-angled triangles indicates the midpoint of the plate width.