A method for manufacturing a highly-crystallized double oxide powder composed of a single crystal phase which can be used as a phosphor material, a dielectric material, a magnetic material, etc. The method involves forming fine droplets of a raw material solution containing a raw material compound that includes at least one metal element and/or at least one semi-metal element that constitutes a double oxide, and heating these droplets at a high temperature, wherein the raw material solution is a solution which exhibits only one main peak attributable to the decomposition reaction of the raw material compound or a reaction intermediate thereof in a DTA profile when the solution is dried and solidified and subjected to TG-DTA measurement.