Two-dimensional electron systems have attracted attention as thermoelectric materials, which can directly convert waste heat into electricity. It has been theoretically predicted that thermoelectric power factor can be largely enhanced when the two-dimensional electron layer is far narrower than the de Broglie wavelength. Although many studies have been made, the effectiveness has not been experimentally clarified thus far. Here we experimentally clarify that an enhanced two-dimensionality is efficient to enhance thermoelectric power factor. We fabricated superlattices of [N unit cell SrTi1-x Nb x O3|11 unit cell SrTiO3]10 - there are two different de Broglie wavelength in the SrTi1-x Nb x O3 system. The maximum power factor of the superlattice composed of the longer de Broglie wavelength SrTi1-x Nb x O3 exceeded 5 mW m-1 K-2, which doubles the value of optimized bulk SrTi1-x Nb x O3. The present approach - use of longer de Broglie wavelength - is epoch-making and is fruitful to design good thermoelectric materials showing high power factor.