A miniaturized voltage-transforming device includes a first circuit board and a second circuit board parallel to and separated from each other by a predetermined distance so that there is no physical connection therebetween, and a transformer having a plurality of primary-side pins and a plurality of secondary-side pins, wherein the transformer is located beside the first circuit board and the second circuit board, and has its primary-side pins and secondary-side pins directly or indirectly connected to the first circuit board and the second circuit board physically, so that the transformer is electrically connected to the first circuit board and the second circuit board via the primary-side pins and the secondary-side pins.