PURPOSE: To prevent crack and to efficiently obtain planar glass of a large shape by using a mold vessel having recesses in the inside bottom at the time of obtaining a dry gel from a wet gel.

CONSTITUTION: The sol soln. poured into the mold vessel 7 having the recesses 1a to 6a in the inside bottom is gelatinized by hydrolysis and dehydration condensation to form the wet gel 8 in a 1st stage. The wet gel 8 shrinks by maintaining the shape approximately resembling the cross sectional shape at the time of the wet gel 8 in a 2nd stage. Spaces 10 are formed between the inside surface of the side walls of the mold vessel 7 and the gel 9 during the course of drying and shrinkage at this time. The bottom ends of the parts 1b to 6b which are heretofore positioned in the recesses 1a to 6a at the time of the wet gel 8 are risen up to the top end of the inside bottom of the mold vessel 7 by drying and shrinkage, by which gaps 11 are formed between the inside bottom of the mold vessel 7 and the gel 9 during the course of drying and shrinking. The gel 9 in the course of drying and shrinking is thereafter dried and shrunk further to form the dry gel 12.