A U-shaped iron core transporting/assembling tank is divided into a first leg iron core tank for housing a first leg iron core, a second leg iron core tank for housing a second leg iron core, and a lower-yoke iron core assembling tank. The first and second leg iron core tanks are integrated to constitute an erection tank. The lower-yoke iron core assembling tank has an opening portion for opening or closing which is formed in one longitudinal side surface of the lower-yoke iron core and through which the lower-yoke iron core can be carried in or out. The lower-yoke iron core assembling tank is connected at the opening portion which is opened when the lower-yoke iron core is housed in the lower-yoke iron core assembling tank.