In the synthesis of the active material, it is preferable to use, for example, a solid phase synthesis method using Nb₂O₅ and TiO₂ as starting materials. The average particle diameter of Nb₂O₅ particles as the starting materials is preferably smaller than that of TiO₂ particles. More preferably, the average particle diameter of Nb₂O₅ particles is 1 μm or less and the average particle diameter of TiO₂ particles is 4 μm or more. Thus, when there is a difference in particle diameter between Nb₂O₅ particles and TiO₂ particles, atomic diffusion during sintering in the solid phase method is not sufficiently activated. The use of this phenomenon allows an Nb rich phase such as Nb₁₀Ti₂O₂₉ or Nb₂₄TiO₆₂ or a TiO₂ rutile phase to be intentionally formed. As the average particle diameter for Nb₂O₅ particles and TiO₂ particles, a particle diameter when the volume frequency is 50% is used. The average particle diameter can be measured with a laser diffraction and scattering-type particle size distribution measuring device.

In the synthesis by the solid phase method, it is possible to perform temporary sintering as the first sintering before the sintering. The temporary-sintering temperature is preferably from 600° C. to 1100° C. A small amount of impurity components (e.g., water and an organic compound) which are adhered to the raw material powder can be removed by the temporary sintering.