Dielectric breakdown is prevented between opposing two semiconductor chips, to improve the reliability of a semiconductor device. A first semiconductor chip has a wiring structure including a plurality of wiring layers, a first coil formed in the wiring structure, and an insulation film formed over the wiring structure. A second semiconductor chip has a wiring structure including a plurality of wiring layers, a second coil formed over the wiring structure, and an insulation film formed over the wiring structure. The first semiconductor chip and the second semiconductor chip are stacked via an insulation sheet with the insulation film of the first semiconductor chip and the insulation film of the second semiconductor chip facing each other. The first coil and the second coil are magnetically coupled with each other. Then, in each of the first and second semiconductor chips, wires and dummy wires are formed at the uppermost-layer wiring layer.