Provided is a process for preparing a homogenous fluoride glass containing high purity BaF₂ through the CVD process characterized in that the used gaseous mixture comprising: a barium β-diketonate complex serving as a first starting material and represented by the following general formula (1) of:
wherein R is an alkyl group having 1 to 7 carbon atoms, R′ is a substituted alkyl group having fluorine atoms substituting hydrogen atoms and represented by C n F 2n+1 where n is an integer of from 1 to 3; and
a gaseous or vaporizable compound of the matallic element constituting said fluoride glass, the gaseous or vaporizable compound serving as a second starting material; and a fluorine-containing gas serving as fluorinating agent. Further provided is a process for preparing a preform for a fluoride optical fiber which is low in trasmission loss, by depositing the fluoride glass over the interior wall of a cylindrical tube or the wall of rod-like glass substrate through the CVD process followed by collapsing.