The validity of the correction for the interaction parameter g of the Flory-Huggins free energy (Langmuir 2003, 19, 5240) was examined by taking the contributions of spatial variation in the volume fraction into account. Developing the statistical-mechanical view of the invariance conjecture and the Landau expansion with the nonanalytic coefficients for the polymer solutions, we showed that the chain-length dependent correction term H in the interaction parameter g was reasonably introduced. We derived the explicit function form of H as a function of chain-length and temperature to be consistent with the critical power laws for both the concentration difference and the diameter in coexisting phases. It was successfully applied to the systems polystyrene in methylcyclohexane and poly(methyl methacrylate) in 3-octanone.