Stability of ferroelectric phase and its thickness-dependence were theoretically investigated by incorporating the semiconductor properties of ferroelectrics into the Ginzburg-Landau (GL) theory, which successfully explained experimental results for ferroelectric oxides, e.g., BaTiO 3 . An expression of the GL energy of a ferroelectric in a complex structure was derived. Using the energy, the semiconductivity was found to suppress the instability more effectively than the inhomogeneity ( P)- and multidomain-effects.