PURPOSE: To prevent cracking in an optical glass source material by coating the inside of a glass vessel with a specified substance at the time of manufacturing the optical glass source material by gelling a colloidal silica soln. contg. colloidal silica and a solvent in a glass vessel.

CONSTITUTION: Colloidal silica sol is mixed with phosphoric acid and stirred to form a mixed sol 3a. This sol 3a is placed in a glass vessel 1 coated with paraffin 2 on the inside, the vessel is covered with a cover 4, tightly enclosed, and allowed to stand at room temp., to gel the sol 3a. Then, the cover 4 is replaced by a cover 5 having a gas releasing hole 6, and the vessel 1 is placed in a thermostat kept at 70°C. Since the paraffin has 60°C melting point, it melts and the moisture contained in the sol 3a is dehydrated and released through the hole 6, and the sol 3a is contracted and become gel 3b. This gel 3b is smoothly dehydrated without generating any crack, and it is heated to 1,000W1,200°C to manufacture an optical glass source material.