NOVELTY - Titanium-dioxide ultrafine particles are obtained by precipitation in a fluid containing titanium compound. The fluid is a thin film fluid formed between processing surfaces (1,2) which mutually oppose and are arranged contactably and separably, with at least one rotating with respect to the other. The ultrafine particles are deposited in the thin film fluid. The method uses a fluid-pressure providing mechanism for providing a pressure to a to-be-processed fluid, two or more processing portions (10,20), and a rotational drive mechanism.    USE - Manufacture of titanium-dioxide ultrafine particles for use in compositions and suspensions (all claimed). Uses include but are not limited to solar cell applications, photocatalyst applications, raw materials of dielectric materials, silicone rubber additives, cosmetics, fuel cells, electronic paper, developing agents used in electrophotography, photochromic particles, and spacers of LCDs.    ADVANTAGE - The method enables efficient mass-production of titanium-dioxide ultrafine particles with high productivity, without product plugging (due to self-exhaustibility) and without requiring high pressure.    DETAILED DESCRIPTION - Titanium-dioxide ultrafine particles are obtained by precipitation in a fluid containing titanium compound. The fluid is a thin film fluid formed between processing surfaces (1,2) which mutually oppose and are arranged contactably and separably, with at least one rotating with respect to the other. The ultrafine particles are deposited in the thin film fluid. The method uses a fluid-pressure providing mechanism which provides a pressure to a to-be-processed fluid, two or more processing portions (10,20) approachable and retreatable on a relative target with respect to processing portion (10), and a rotational drive mechanism in which processing portions (10,20) are rotated relatively. At least two processing surfaces (1,2) are provided in a position which mutually opposes in each processing portion. Each processing surface comprises a portion of sealed flow path where the to-be-processed fluid of specified pressure is poured. Processing portion (20) is equipped with a pressure surface, and portion(s) of the pressure surface is comprised by processing surface (2). A force moved to the direction which receives the pressure which the mechanism provides to a to-be-processed fluid, and makes processing surface (II) separate the surface, is generated. The to-be-processed fluid is passed between the processing surfaces which are rotated approachably-retreatably and relatively. While the to-be-processed fluid forms a liquid film, it passes between the processing surfaces.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the apparatus used for manufacturing titanium-dioxide ultrafine particles.    Processing surfaces (1,2)    Processing portions (10,20)    Holders (11,21)    Ring accommodating portion (41)    Rotating shaft (50)