The dielectric constant depending on the film thickness for SrTiO3 films formed on Ru was investigated after an annealing step at 600 degrees C, which shows that the dielectric constant increased abruptly with the film thickness up to 20 nm and then increased slightly, remaining relatively constant at a value of about 65. The abrupt increase was due to the crystallinity of SrTiO3 films. On the other hand, the slight increase was related to the existence of nonstoichiometric region near the interface of SrTiO3 film and Ru, which was intermixed with SrTiO3 and Ti-O phases having an equivalent oxide thickness over 0.32 nm.