We have explored dielectric properties of ultra-thin Si(111) films using two different evaluation methods based on the first-principles calculations, the internal field (IF) method and the dipole moment (DM) method. As the thickness of the film increases, the dielectric constant evaluated at the innermost region of the film approaches a value near to the experimental bulk dielectric constant at a thickness of only 8 bi-layers. The theoretical value of the electronic dielectric constant for the Si(111) film is 12.85, which is only 6.2% higher than the experimental one. Furthermore, we have shown that the IF method is applicable to microscopic analysis of dielectric properties. The spatial variation in dielectric constant near the surface reveals that depolarized charges at the surface penetrate into the film up to the thickness of 3 bi-layers, resulting in an effective reduction of the dielectric constant near the surface. We have also discussed a microscopic picture of polarization on atomic scale.