A first drill is pressed, in a rotating manner, against a lower surface of a glass substrate, to thus make a first hole of a predetermined depth. A second drill is pressed, in a rotating manner, against a position on an upper surface of the glass substrate opposing the first hole, to thus make a second hole, and the first hole and the second hole are brought into mutual communication with each other, to thus make a through hole in the glass substrate. A step on an internal periphery of the through hole formed as a result of the first hole and the second hole overlapping each other in a thicknesswise direction of the glass substrate is situated on the upper surface side than to the center of the glass substrate achieved in its thicknesswise direction.