PURPOSE: To remarkably decrease iron loss and power loss which is caused by an excitation current and obtain an assembled-type transformer which can be installed in flat space by stacking unit transformers on top of each other with spacers interposed between their yokes and holding the unit transformers with a through stud piercing the yokes.

CONSTITUTION: Unit transformers 1 are integrated by using holding plates 15 and 16 to cover the central openings 13 and 14 of yokes 10 and 11, a through stud 17 passed through the central holes of the holding plates 15 and 16, and nuts 18. The unit transformers 1 are stacked on top of each other with piling spacers 22 interposed therebetween, held and tightened with the through stud 17 for integration, and installed in a switch-board together with a pair of primary electrodes 24 and a pair of secondary electrodes 25. In the switch-board, lead wires 20 at both ends of the outer windings 6 of the unit transformers 1 are connected to the electrodes 24 in parallel and their inner windings 5 are connected to the electrodes 25 in parallel. The number of the stacked unit transformers 1 is determined according to maximum load power.