PURPOSE: To improve mold releasability, to prevent burring and to obtain an optical element excellent in profile irregularity and dimensional accuracy by filling molten tin in a glass forming mold and then supplying molten glass while discharging the molten tin.

CONSTITUTION: A mold 1 is heated to 300-700°C, the outlet 3 is closed in a nonoxidizing atmosphere, and molten tin 5 is filled into the mold 1 from an inlet 2. A control stop valve 8 is provided close to the outflow orifice of a molten tin vessel 10 (molten glass vessel) to control the outflow of tin or glass from the vessel when the materials are not supplied. The mold is then heated by a heater, molten glass 11 is supplied, and the outlet 3 is simultaneously opened to discharge the molten tin 5. When the molten glass 11 is supplied into the mold, the glass is floated on the boundary of the molten tin 5 since glass is lower in sp.gr. than tin, and the glass level is kept horizontal. A pressure is exerted on the mold and the glass is uniformly filled in the mold by controlling the discharge of the molten tin 5 when the molten glass 11 is filled into the mold.