An improvement is achieved in the reliability of a semiconductor device by preventing a dielectric breakdown between two semiconductor chips facing each other. During the manufacturing of first and second semiconductor chips, the process of planarizing the upper surfaces of insulating films is performed. Then, the first and second semiconductor chips are stacked via an insulating sheet with the respective insulating films of the first and second semiconductor chips facing each other such that the respective coils of the first and second semiconductor chips are magnetically coupled to each other.