PURPOSE: To simply form a hard edge part at low cost by performing spark discharge by approaching or contacting a negative electrode of an electric power supply source to or with an edge part of a positive electrode.

CONSTITUTION: A circular saw 10 composed of carbon tool steel is connected to a positive electrode wire of a capacitor 12 through a transformer 11. On the other hand, in a negative electrode of this capacitor 12, a terminal 13 is arranged on the tip of the conductor. The terminal 13 of the negative electrode is approached to or contacted with the edge side of a saw edge 10a in the circular saw 10. Thereby, a part performing spark discharge becomes a corrosive part 14 of the steel. In this way, the spark discharge is performed in the whole saw edge 10a. Next, a sawtooth is set in the corrosive part 14 of the saw edge 10a by a sawtooth setting machine, and when it is further polished by a polisher, a hard saw edge can be formed. A position to perform this spark discharge is not limited to the edge side of the saw edge 10a, and is may be alternately performed on one side surface of the saw edge 10a and the other side surface on the opposite side, and the spark discharge may be performed on both side surfaces of the saw edge 10a.