To prevent the formation of split fins on through holes having C surface portions which overlap split grooves and the occurrence of conductor striping at through hole areas when a substrate is split after the formation of conductors, and lower the height of swells which are formed on the surfaces of the rims of the through holes during ceramic molding.

A ceramic substrate 1 is molded with a punch 5 for perforating having a cutting edge 5c for split groove formation on a C surface portion 5b and a die having a cutting edge 6 for split groove formation, and fired. Split grooves 3 are formed to the C surface portions 2a of through holes 2, and the depth D1 of the split grooves 3 is identical with or greater than the depth D2 of the C surface portions 2a.