Provided is a U-shaped iron core transfer/assembling method, by which a first leg iron core, a second-leg iron core and a lower-yoke iron core are housed in a first leg iron core tank, a second leg iron core tank and a lower-yoke iron core tank, respectively, and those iron cores are individually transported in sideways attitude. Next, the lower-yoke iron core is transferred into a lower-yoke iron core assembling tank. While the first leg iron core, the second leg iron core and the lower-yoke iron core being kept in the tilted postures, the first leg iron core tank, the second leg iron core tank and the lower-yoke iron core assembling tank are individually opened at their openings and are jointed to each other.