PURPOSE: To produce fine powder of SiO2 having a definite particle diameter and specific surface area, at a low temperature, by irradiating a gaseous mixture of a silane gas and ozone with ultraviolet radiation.

CONSTITUTION: A silane gas diluted with a rare gas or N2 gas to a concentration lower than explosion limit is mixed with ozone at normal temperature and pressure and irradiated with ultraviolet radiation of 200W320nm wavelength. Since the growth of particle proceeds by the collision of activated intermediate against the surface of particle in place of mutual cohesion of critical nuclei, the size and specific surface area of particle can be controlled by selecting the reaction condition.