Substrates suitable to manufacture and products of a thin film semiconductor device are provided, by at first preparing a manufacturing substrate having a characteristic capable of enduring a process for forming a thin film transistor and a product substrate having a characteristic of being suitable to direct mounting of the thin film transistor in a preparatory step, then applying a bonding step to bond the manufacturing substrate to the product substrate for supporting the product substrate at the back, successively applying a formation step to form at least a thin film transistor to the surface of the product substrate in a state reinforced with the manufacturing substrate and, finally, applying a separation step to separate the manufacturing substrate after use from the product substrate.