PURPOSE: To synthesize fine particles having ≤0.1μm particle size under normal pressure by mixing reactant gas with gas which is inactive to laser in the atmosphere of the inactive gas under normal pressure to dilute the reactant gas, and feeding the gaseous mixture to a zone irradiated with laser light synthesizing fine powder in the gaseous phase.

CONSTITUTION: Reactant gases fed through flow meters 1a, 1b are mixed with inactive gas to laser light which is fed through a flow meter 1d and a line 2b in a line 2 and diluted. Thus, diluted state of the reactant gas is formed in a reactor 6 held at normal pressure or near the normal pressure. The reaction vessel is irradiated with laser light to cause gaseous phase synthesis. Further, not only fine powder can be formed but also the control of the particle size of formed fine particles is possible by changing the proportion of the reactant gas fed through flowmeters 1a, 1b to the amt. of the inactive gas fed through the flowmeter 1d.