There are provided a method of superflattening an oxide crystal that is soluble neither with acid nor with alkaline, a method of making a ReCa₄O(BO₃)₃ family oxide single crystal thin film using the superflattening method, a ReCa₄O(BO₃)₃ family oxide single crystal thin film having a SHG property, a superflattening method for light incident/emitting surfaces, and a defect assessing method for oxide crystals. The surface of an oxide crystal that is soluble neither with acid nor with alkaline is reduced with a reducing agent, the reduced oxide crystal surface is dissolved with an aqueous solution of acid or alkaline, the surface dissolved oxide crystal is heat-treated in the atmosphere, whereby the surface of an oxide crystal that is soluble neither with acid nor with alkaline is superflattened to an atomic level. According to this method, a chemically stable oxide which because of its complexity in both composition and structure is soluble neither with acid nor with alkaline and is insoluble even with a fluoric acid is allowed by reduction to be converted into a simpler oxide conventionally soluble with hydrochloric, nitric or sulfuric acid; hence a surface of its crystal is rendered capable of dissolving. Then, heat-treating the dissolved surface in the atmosphere at a suitable temperature for a suitable time period allows surface atoms to be rearranged and the surface to be superflattened to an atomic level. The present invention is applicable to the technical fields that require ultraviolet laser light, especially as core technologies of optical devices applied to optical information processing, optical communication or the like.