In the synthesis by the solid phase method, the sintering temperature as the second sintering is preferably from 1300 to 1400° C., more preferably 1350° C. The sintering temperature is set to the range of 1300 to 1400° C., whereby the grain growth of crystallites in the plane direction of the peak appearing at 2θ of 26.0±0.1° progresses. Thus, if the crystallites anisotropically grow, a binding interface between the Nb-rich phase and the Nb₂TiO₇ phase can be formed smoothly. When the sintering is performed at a temperature lower than 1300° C., the anisotropic grain growth does not progress. Further, when the sintering is performed at a temperature exceeding 1400° C., diffusion of Nb and Ti elements progresses rapidly. Accordingly, the composition of the material becomes homogeneous easily, and it becomes difficult to form a mixed phase with an Nb-rich phase such as Nb₁₀Ti₂O₂₉ or Nb₂₄TiO₆₂.

Annealing the oxide can be performed after the sintering. The annealing temperature is preferably from 600° C. to 1000° C. If the annealing is performed in the temperature range, the Nb rich phase can be reduced.

Rapid cooling the oxide can be performed after the sintering. As a result, the Nb rich phase can be increased.