First, raw material powder (LTO powder) composed of lithium titanate Li₄Ti₅O₁₂ is prepared. Commercially available or newly synthesized LTO powder may be used as the raw material powder. For example, powder obtained by hydrolyzing a mixture of titanium tetraisopropoxy alcohol and isopropoxy lithium may be used, or a mixture containing lithium carbonate, titania, or the like may be fired. The raw material powder preferably has a volume-based D50 particle size of 0.05 to 5.0 μm, more preferably 0.1 to 2.0 μm. A larger particle size of the raw material powder tends to increase the size of the pores. Further, in the case where the particle size of the raw material is large, milling (such as pot milling, bead milling, and jet milling) may be performed to a desired particle size. The raw material powder is mixed with a dispersive medium and any additive (e.g., binder, plasticizer, and dispersant) to form a slurry. A lithium compound (e.g., lithium carbonate) in an excess amount of about 0.5 to 30 mol % other than LiMO₂ may be added to the slurry to promote grain growth and compensate for a volatile component in a firing process described later. The slurry preferably contains no pore-forming agent. The slurry is defoamed by stirring under reduced pressure, and the viscosity is preferably adjusted into 4000 to 10000 cP. The resultant slurry is formed into a LTO-containing green sheet. The resultant green sheet is in a form of independent sheet. An independent sheet (also referred to as a “self-supported film”) refers to a sheet (including flakes having an aspect ratio of 5 or more) that can be handled in a singular form independently apart from a support that is different therefrom. In other words, the independent sheet does not refer to a sheet that is fixed to a support that is different therefrom (such as a substrate) and integrated with the support (so as to be inseparable or hard to separate). The sheet can be formed by any known process and is preferably formed by a doctor blade process. The thickness of the LTO-containing green sheet may be appropriately selected so as to give the above desired thickness after firing.