PURPOSE: To improve productivity by providing a flow hole for cooling water in an electrode base for holding a bottom electrode, cooling the bottom electrode and thereby greatly prolonging its service life.

CONSTITUTION: In order to cool the bottom electrode 20, a flow hole 38 is pierced, which penetrates through the center of the electrode base 22 as well as the electrode block 28, as a passage for cooling water, and the cooling water coming out of the flow hole 32 of the electrode block 28 is passed to the flow hole 38 of the electrode base 22 by means of a hose 39. As a result, the electrode base 22 for the bottom electrode 20 not cooled by water heretofore is directly cooled, reising the cooling effect of the bottom electrode 20. Consequently, uneven wear, deformation, burr generation, etc., are reduced in the tip holding hole of the bottom electrode 20, greatly prolonging the service life of the bottom electrode 20 till it is exchanged, and enhancing productivity.