PURPOSE: To make the assembly of a transformer easier and improve the stability of insulation by a method wherein a plurality of unit transformers are connected in parallel to compose one single-phase transformer and three such single- phase transformers are arranged to compose one three-phase transformer and the transformer is connected to a gas-insulated bus-bar.

CONSTITUTION: A high voltage connecting lead 14 and a medium voltage connecting lead 15 of each unit transformer are connected to a high voltage bushing 16 and a medium voltage bushing 17 provided to the longitudinal end of each unit transformer along its longitudinal direction through a high voltage lead duct 28 and a medium voltage lead duct 29 respectively. A tertiary lead and a neutral lead are connected to a loaded voltage regulator 6AB provided to the side of the tank along the longitudinal direction through a low voltage lead duct 30 provided on top of the unit transformers opposite to the high voltage lead duct 28. The high voltage bushing 16 and the medium voltage bushing 17 are placed in a high voltage bushing pocket 31 and a medium voltage bushing pocket 32 attached to the sides of the tank respectively. With this constitution, a degree of freedom of the attaching direction of the bushing is increased and it is effective for saving a space of a substation.