A process for producing an ultrafine mixed-crystal oxide characterized by producing an ultrafine mixed crystal oxide comprising primary particles in a mixed crystal state with a BET specific surface area of 10 to 200 m2/g, comprising the step of subjecting a halogenated metal to high temperature oxidation with an oxidizing gas to produce a metal oxide by a vapor phase production method, wherein said halogenated metal is in the form of a mixed gas (a mixed halogenated metal gas) comprising at least two compounds having a different metal elements selected from the group consisting of chlorides, bromides, and iodides of titanium, silicon, and aluminum, and said mixed halogenated metal gas and said oxidizing gas are independently preheated to 500null C. or more prior to a reaction, a ultrafine mixed crystal oxide obtained by the process, and use of the oxide.