PURPOSE: To obtain the titled oxide having uniform particle size distribution and producible with simple process in high yield, by adding a metal alkoxide or an orthosilicic acid alkyl ester to a specific W/O-type microemulsion phase and hydrolyzing the mixture.

CONSTITUTION: Ultrafine particles of metal or silicon oxide having particle diameter of ≤1,000 and adsorbed or attached with a surfactant can be produced by adding (A) a metal alkoxide (e.g. alkoxide of Al, Zr, etc.) or (B) an orthosilicic acid alkyl ester (e.g. ethyl orthosilicate) to (C) a W/O-type microemulsion phase consisting of 3-components comprising (a) a water-soluble or oil-soluble surfactant of formula IWIV [R1 is 6W12C unsaturated or side-chain alkyl; M is alkali or alkaline earth metal; R2 is 6W20C alkyl; X is halogen; R3 is 4W8C alkyl; Z is alkali metal or alkaline earth metal salt of SO3H, OSO3H or COOH], (b) water and (c) a nonpolar organic solvent such as cyclohexane or 4-components comprising the components (a), (b), (c) and (d) an alkanol and hydrolyzing the mixture optionally in the presence of a catalyst.