Thermoelectric device is one of the key devices for the sustainable energy technology. Although semiconductor-based devices are widely used, they sometimes contain toxic, volatile, or rare elements. Very recently, we succeeded in reducing the thermal conductivity at room temperature substantially in a SrTiO3-based material by element substitutions, which makes this material a promising candidate of an n-type oxide thermoelectric device free from the above mentioned problems. This proposal is to measure phonons in this material by inelastic neutron scattering to find anomalies related with the reduction of thermal conductivity and elucidate its origin. We have already obtained a clue with powder samples, and in this proposal we shall obtain further convincing data with a single crystal sample.