Bus bars 16(A to D) and 17(A to C) on the two faces of an insulating plate 15 can be electrically connected by arranging the longitudinal bus bars 17(A to C) on one face of the insulating plate 15 and the transverse bus bars 16(A to D) on the other face of the insulating plate 15, by fitting recesses 16a to 16d of the bus bars 16(A to D) of at least the one face at the suitable positions of the intersecting portions of the bus bars 16(A to D) and 17(A to C) of the two faces in through holes 15a of the insulating plate 15 to bring the recesses 16a to 16d into contact with the bus bars 17(A to C) of the face, as confronted thereby, and to weld them 16a to 16d to the same bus bars 17(A to C).