To provide a method for boring a hole having a circular, elliptic, or elongated shape and tapered in a horn shape by electrical discharge machining using a small-diameter electrode.

In this electric discharge machining, a C-axis table on which a workpiece is placed, an XY-axis stage, and a Z-axis electrode head are numerically controlled in synchronism with each other. The Z-axis electrode head includes an axially feeding device and a mechanism for tilting the axial direction in one plane. The distance between the designated boring point of the workpiece attached to the C-axis table and the center of the C-axis table is set at R. While moving the Z-axis feeding device by a reciprocating shuttle motion, the circular tapered hole having a profile cross section of horn shape is electrical discharge-machined by synthesizing the synchronous motions calculated by using the swing angle of a C-axis and the following formulas [expression 2], [expression 3], or [expression 4] of the XY stage. The formula [expression 2]: Ci=i, Xi=Rsini, Yi=Rcos, the formula [expression 3]: Ci=i, Xi=Rsini, Yi=Rcosi+, and the formula [expression 4]: Ci=i, Xi=Rsini, Yi=Rcosi+qi.