PURPOSE: To prepare the titled ultrafine particle by forming dust cloud of metal powder of a metal which composes a part of the composition of the target ceramic material of the ultrafine particle in reactant gas contg. elements which form other part of the composition of the ceramic ultrafine particle, igniting the powder dust cloud to cause defragration.

CONSTITUTION: Reactant gas (e.g. air) contg. an element (e.g. O) for forming a part of the components of the target ceramic ultrafine particle is filled in a closed vessel 1, and metal powder 3 (e.g. Al) which forms another part of the components of the target ceramic particle is charged to a hopper 4. The bottom of the hopper 4 is opened, and compressed air is fed from a feed pipe 5, and a solenoid valve 9 is opened for a short while. Thus, the powder 3 is fed to a conical section 2b of a defragration device 2 by the compressed air, and scattered in the device 2 forming dust cloud. The dust cloud is ignited by the spark generated by discharge electrodes 7 to cause degragration forming ceramic ultrafine particles such as alumina. By this process, ceramic ultrafine particle is prepd. efficiently and inexpensively.