Oil-filled transformer and production method for tank used therein

The objective of the invention is to provide an oil-filled transformer wherein the heat-dissipating characteristics and the strength of the tank, and a size reduction of the oil-filled transformer, have been taken into consideration. In order to attain this objective, the oil-filled transformer which is filled with an insulating oil houses an iron core-coil assembly assembled from an iron core and a coil inside the tank, wherein the tank has a bottom plate, a flange for mounting a lid, and a trunk section comprising a side plate located between the flange and the bottom plate, the trunk section having a plurality of embossed parts formed on the surface thereof, and the trunk section being constituted by welding the side plate having the embossed parts formed thereon into a cylindrical shape, wherein a weld section is constituted by welding in such a manner as to overlap some of the embossed parts.