PURPOSE: To produce a high-purity slica-based glass doped with F as an optical fiber preform by premixing hydrofluoric acid and a specified base, adding the mixture to a mixture contg. a silicon alkoxide, hydrolyzing and gelling the obtained mixture, drying and calcining the gel.

CONSTITUTION: HF and an amine-based base selected from a group consisting of piperidine, pyrrolidine, piperazine, pyridine, isopropylamine, N-ethyl-n- propylamine, n-propylamine, isopropylamine and diethylamine are mixed in 2-5 molar ratio of the HF to the base to obtain a homogeneous soln. A silicon alkoxide consisting of an oligomer, ≥30% of which consists of a polymer having >3 average polymerization degree, is added to the soln., and the mixture is hydrolyzed, gelled, then gradually dried and integrated to obtain a dry gel which is further heated in an oxygen atmosphere to remove the remaining org. matter. The gel is heated to high temp. in an He atmosphere to produce a transparent glass contg. F and appropriate as a glass fiber preform.