When a thin film is formed on a flexible and filmy substrate by a vapor phase method, the substrate is prevented from warping to be caused by the internal stress remaining in the thin film. When the thin film is formed by the vapor phase method, the substrate is previously curved so that the stress acts in the direction canceling the internal stress remaining in the thin film to be formed prior to the filming. Accordingly, the stress of the curved substrate cancels out the stress remaining in the thin film formed on the substrate. The substrate having a thin film formed thereon is not warped, the stress in the interface between the thin film formed and the substrate is removed, and the thin film has no cracks to be caused by the stress.