A method for manufacturing a flexible semiconductor device is disclosed. The method includes: forming a separation layer of a metal over a substrate; treating the separation layer with plasma under an atmosphere containing nitrogen, oxygen, silicon, and hydrogen; forming a layer over the plasma-treated separation layer, the layer being capable of supplying hydrogen and nitrogen to the separation layer; forming a functional layer over the separation layer; performing heat treatment to promote the release of hydrogen and nitrogen from the layer; and separating the substrate at the separation layer. The method allows the formation of an extremely thin oxide layer over the separation layer, which facilitates the separation, reduces the probability that the oxide layer remains under the layer, and contributes to the increase in efficiency of a device included in the functional layer.