Abstract: Provided is a complex oxide having high density and high lithium ion conductivity and low activation energy. The complex oxide has a chemical composition represented by Li4-xSr2-xLaxZrO6 (0?x?1.0) and belongs to a monoclinic space group P21/n. The relative density of this complex oxide can be made to be 100%. The lithium ion conductivity of this complex oxide can be made to be 6.0×10?4 S/cm or more. This complex oxide is produced by melting at least a part of a raw material having a chemical composition represented by Li(4-x)ySr(2-x)zLaxZrO6 (0?x?1.0, 1<y and 1<z) to form a molten portion and moving the molten portion at a movement speed of 8 mm/h or faster.