A resin layer made of thermoplastic resin is formed on a supporting substrate, and then, an insulating layer is formed on the first resin layer. Then, an interlayer connector is formed through the insulating layer and then, a wiring layer is formed on the first resin layer so as to be electrically connected with the interlayer connector. Thereafter, a first semiconductor chip is mounted on the wiring layer. Then, the first resin layer is heated so that the supporting substrate and the insulating layer are relatively shifted one another to shear the first resin layer, thereby separating the supporting substrate and the insulating layer and forming a semiconductor device.