PURPOSE: To make uniform the cross-section of an electrode section while to improve the focus characteristic and the assembling workability, by additionally providing a belelling process between a boring process for burring and a forming process for burring.

CONSTITUTION: A hole 12 having predetermined inner diameter d1 is made through a material board 10 through punching, then an extended section 14 is formed through extending work 13 and a hole 16 for burring having the inner diameter of d3 is formed through punching 15. Thereafter a bevel section 31 is formed on the end face at the inside of said hole 16 through coining work 30. A coining punch 32 is employed in said work 30 to provide an arched or tapered bevel section 31. The range of said bevel section 31 is such that the ratio between the bevel depth (t) and the depth of the material at the edge section of hole or t/T=1/3. Thereafter a cylindrical section 18 is formed through burring work 17. Consequently the focus characteristic and the assembling workability are improved considerably.