Thermoelectric energy conversion attracts increasing attention as a technology for effectively reusing waste heat. Although thermoelectric materials that show a good thermoelectric figure of merit (ZT) have been proposed thus far, they are not practical at all because they are thermally and chemically unstable and composed of toxic elements. In order to address this issue, the author focused on metal oxides as thermoelectric materials that are thermally and chemically stable and non-toxic, and succeeded in significantly improving thermoelectric ZT by using two-dimensional electron gas and elemental substitution. In 2007, the author focused on Prof. Dresselhaus’s theory that “by confining carriers in a quantum well thinner than the thermal de Broglie wavelength, the thermoelectric power can be greatly enhanced without lowering the conductivity”, and the oxide superlattice was introduced. In 2010, the author demonstrated a field-effect transistor structure on an insulator SrTiO₃ crystal and measured thermopower while inducing 2DEG with a thickness of 2 nm by applying a voltage, similar to an artificial superlattice. It was discovered that the thermoelectric field can be increased 5 times as much as the bulk ratio. Furthermore, in 2020, the author found that the Ba_1/3CoO₂ thin film with a layered crystal structure had the highest room temperature ZT of 0.11 among metal oxides.