PURPOSE: To manufacture a base material for an infrared optical fiber having a core-clad waveguide structure, by erecting a fine wire at the center of a casting mold, pouring a melt for a clad, pulling out the wire, and filling molten glass for a clad into the resulting small cavity.

CONSTITUTION: A casting mold is composed of an outer frame 1 divided into a plurality of parts 2, 2', 2" and a ring 3 for clamping the frame 1. A fine metallic wire 6 of platinum or the like is erected at the center of the inner hollow 5 of the mold with a bottom cover 4 in-between, and molten glass for a clad is poured into the hollow 5. The wire 6 is heated beforehand by directly supplying an electric current or by other method, and it is pulled out immediately after pouring the molten glass. Molten glass for a core is filled into the resulting small cavity and annealed so as to prevent the production of stress. After finishing the molding, the parts 2, 2', 2" are detached to obtain a base material for an infrared optical fiber. By this method, a fluoride glass base material for an optical fiber capable of transmitting infrared rays having about 2W6μm wavelengths can be manufactured.