a welding transformer for use in a resistance welding machine has primary and secondary coils . the primary coil includes a plurality of spaced juxtaposed coil units mounted on a base board . each of the coil units includes a plurality of juxtaposed coil unit elements with insulating members interposed therebetween . each of the coil unit elements includes a plurality of turns of a conductive member , and a plurality of conductors mounted on the base board and electrically interconnecting the coil unit elements . a pair of cores is combined with the coil units .