A transformer transporting/assembling method comprises a transportation step of transporting a U-shaped iron core with an iron plate horizontally extending to a place near an installation site and an erection step of erecting the U-shaped iron core together with an erection tank from a state that the iron plate extends horizontally to a state that the iron plate extend vertically, with the U-shaped iron core contained in the erection tank. The erection step includes a sub-step in which a portal lifter composed of two booms which are vertically expandable and parallel movable with a predetermined distance therebetween and between which the erection tank is disposed and a beam horizontally connecting the two booms and vertically moved as the booms expands or contracts is used, the beam supports the erection tank through a flexible member at predetermined support portions, and the beam is vertically moved by expanding/contracting the booms.