PURPOSE: To obtain fluoride glass suited to optical fiber having low content of impurities having OH groups by dehydrating starting materials for fluoride glass in a low humidity atmosphere by heating, halogenating and then heat-melting, and casting the starting material in a preheated mold.

CONSTITUTION: A closed system comprising an atmosphere furnace 2, an annealing furnace 2 and a casting box 1 is constructed. Low humidity gas having ≤-20°C dew point of moisture is introduced from inlets 7 and 9 of low humidity gas and starting materials for rhe fluoride glass are charged to a crucible 12. A halogenating agent such as NH4.HF, etc. is further added thereto, and the crucible is placed on a vertically movable crucible bed 14. The crucible 12 is moved upward in the atmosphere furnace 2 to heat and dehydrate the material in the crucible, and the material is heated further to halogenate the OH groups and oxides contained in the starting material for glass. After melting the starting material for the fluoride glass successively, the heated product is casted in a casting mold which has been preheated in an annealing furnace 3 and quenched. By this process an, aimed fluoride glass is obtd.