PURPOSE: To eliminate the need for grinding and polishing without deteriorating the optical function by softening a sized cubic or rectangular parallelopiped glass material upon heating, rounding the obtuse-angle part by surface tension and smoothing the surface.

CONSTITUTION: A square bar-shaped or plate glass material is cut to a sized cubic or reactangular parallelopiped glass blank 17 having the volume V=a×b×c almost equal to the volume of an optical element to be formed. The blank 17 is placed on a holder 16 provided in a housing 19 in a heating furnace, heated to 800-850°C by a heater 18 furnished adjacent to the inner wall of the housing 19 and softened, the obtuse-angle part is rounded by the surface tension of the blank 17, and the surface roughness Rmax is controlled to ≤0.04μm.