A transformer comprising: - a first iron core group including a plurality of iron core stacks adjacent to each other; - a second iron core group disposed to face the first iron core group while being spaced therefrom, the second iron core group including a plurality of iron core stacks each disposed to face a corresponding one of the plurality of iron core stacks of the first iron core group; and - a plurality of winding portions each wound across a corresponding one of the plurality of iron core stacks of the first iron core group and a corresponding one of the plurality of iron core stacks of the second iron core group, the corresponding one of the plurality of iron core stacks of the second iron core group facing the corresponding one of the plurality of iron core stacks of the first iron core group, - wherein the plurality of iron core stacks of the first iron core group and the plurality of iron core stacks of the second iron core group each include a plurality of annular iron cores stacked on each other, characterized in that each of the plurality of winding portions includes a pair of low-voltage windings and a high-voltage winding sandwiched between the pair of low-voltage windings, wherein the high-voltage winding is spaced from each of the pair of low-voltage windings, wherein an insulation distance is maintained between each of the pair of low-voltage windings and a corresponding one of the iron core stacks (10, 11), the insulation distance corresponding to a difference between a potential generated in each of the pair of low-voltage windings and a potential present in the corresponding one of the iron core stacks (10, 11), wherein an insulation distance is maintained between the high-voltage winding and each of the iron core stacks (10, 11), the insulation distance corresponding to a difference between a potential generated in the high-voltage winding and each of the iron core stacks (10, 11), and wherein an appropriate insulation distance is maintained between each of the pair of low-voltage windings and the high-voltage winding, the appropriate insulation distance corresponding to a difference between a potential generated in each of the pair of low-voltage windings and a potential generated in the high-voltage winding. - a first iron core group including a plurality of iron core stacks adjacent to each other; - a second iron core group disposed to face the first iron core group while being spaced therefrom, the second iron core group including a plurality of iron core stacks each disposed to face a corresponding one of the plurality of iron core stacks of the first iron core group; and - a plurality of winding portions each wound across a corresponding one of the plurality of iron core stacks of the first iron core group and a corresponding one of the plurality of iron core stacks of the second iron core group, the corresponding one of the plurality of iron core stacks of the second iron core group facing the corresponding one of the plurality of iron core stacks of the first iron core group, - wherein the plurality of iron core stacks of the first iron core group and the plurality of iron core stacks of the second iron core group each include a plurality of annular iron cores stacked on each other, each of the plurality of winding portions includes a pair of low-voltage windings and a high-voltage winding sandwiched between the pair of low-voltage windings, wherein the high-voltage winding is spaced from each of the pair of low-voltage windings, wherein an insulation distance is maintained between each of the pair of low-voltage windings and a corresponding one of the iron core stacks (10, 11), the insulation distance corresponding to a difference between a potential generated in each of the pair of low-voltage windings and a potential present in the corresponding one of the iron core stacks (10, 11), wherein an insulation distance is maintained between the high-voltage winding and each of the iron core stacks (10, 11), the insulation distance corresponding to a difference between a potential generated in the high-voltage winding and each of the iron core stacks (10, 11), and wherein an appropriate insulation distance is maintained between each of the pair of low-voltage windings and the high-voltage winding, the appropriate insulation distance corresponding to a difference between a potential generated in each of the pair of low-voltage windings and a potential generated in the high-voltage winding.