A transformer of the invention includes a core which includes a main leg, a side leg, and a yoke, and a winding wire (5) which is wound around the main leg of the core, inside a tank (40) which is filled with an insulating medium, the main leg of the core includes a main wound core (1) having a square cross-section which is formed by winding the amorphous magnetic ribbon and a sub wound core (2) constituted by silicon steel plates which are stacked with curvatures provided at four sides thereof, and a cross-section of a part, where the winding wire (5) is disposed, in the main leg is approximately a circular shape.