To provide a composite oxide powder for a solid oxide fuel cell containing lanthanum, strontium and/or calcium, manganese and oxygen and having a highly uniform composition, and its production method. A composite oxide powder containing lanthanum, strontium and/or calcium, manganese and oxygen, wherein the coefficient of variation (α) of lanthanum is at most 6.0% and the coefficient of variation (β) of manganese is at most 13.0%, as calculated from the peak area ratio of the Lα ray of lanthanum and the Kα ray of manganese measured by an energy dispersive X-ray spectrometer attached to a scanning electron microscope.