PURPOSE: To eliminate the need for polishing a lens surface and to produce a germanium lens with high efficiency by melting a germanium material, casting the melt thereof into a lens casting mold finished to a specular surface and cooling the melt.

CONSTITUTION: The air atmosphere in a chamber is replaced with nitrogen and a specified amt. of the purified Ge material is heated and melted in a nitrogen atmosphere. The liquefied Ge material is then cast into the lens casting mold which is heated to the same temp. and is finished to the specular surface. The gaseous nitrogen is evacuated to reduce the degree of vacuum down to ≤10-1Pa so that the Ge liquid is defoamed. After the liquid is held in this state for the prescribed time, a plunger die finished to a specular surface is pushed into the casting mold and the temp. is gradually lowered until the room temp. is restored. The molded Ge material is then taken out. The Ge lens which does not require surface polishing is thereby produced with the high efficiency and the production cost is reduced.