To provide a forming method for a through hole not requiring high accuracy on the upper face of a rectangular cylinder in a short time and with a low cost.

The boring method includes a process for fitting a flat face side of a tip end 3a of a bore opening tool to a press directing the flat face side in the longitudinal direction of a rectangular cylinder, a process for inserting a rectangular column into the rectangular cylinder, a process for positioning a groove formed on the rectangular column on the boring position of a through hole of the rectangular cylinder, a process for fixing the boring position of the through hole of the cylindrical column directly below the boring tool fitted to the press, and a process for lowering the bore opening tool to the groove of the rectangular column penetrating the rectangular cylinder, by using the rectangular column 1 boring the through hole, the rectangular column 2 for inserting into the rectangular cylinder, the groove 2a formed on the rectangular column, the press, and the bore opening tool 3 to be fitted to the press.