PURPOSE: To shorten the pressurizing cycle time by preheating a casting mold cavity and glass preform, then heating both to a temp. at which a specific glass viscosity is attained, pressing the preform in an isothermal state and taking the molding as a lens from the cavity.

CONSTITUTION: The glass preform having a volume equal to the volume of the desired lens and having a shape approximating the shape of the lens is formed after casting of bar-shaped glass from a glass melt of a prescribed compsn. The casting mold having the exact internal shape complying with the shape of the desired lens is formed by using a stainless steel and is arranged in a globe box filled with a reducing gas. Next, the glass preform is placed into the casting mold and is preheated by an induction coil to the temp. at which the glass viscosity of about 108 to 1010 poises is attained. The glass preform is maintained for a prescribed time at this temp. until a thermal equil, is attained. The glass preform is then pressurized. The glass preform is thereafter cooled down to the temp. at which the glass viscosity of about 1011 to 1012 poises is attained. The load is removed and the molding is taken out of the casting mold and is arranged on a ceramic plate. The molding is subjected to an annealing treatment at this temp. and is then cooled down to room temp., thereby, the glass lens is obtd.