PURPOSE: To provide the titled composition capable of manufacturing economically and efficiently a glass filament having sufficient alkali resistance and mechanical strength by incorporating an appropriate amt. of ZrO2 into silicon- manganese slag.

CONSTITUTION: An alkali resistance improver consisting of ZrO2 is added to the slag obtained as a by-product from silicon-manganese smelting to obtain a starting material composition for the alkali resistant glass fiber contg. ≤15wt% Zr and ≥85% said slag. An alkali resistant glass fiber suitable for reinforcing cement is obtained by spinning said composition into a glass filament. The C contained in said silicon-manganese slag is preferably oxidized by blowing air in, etc. to CO or CO2 before or after the addition of ZrO2 and removed, and the Fe is preferably oxidized to an oxide. An alkali resistance improver contg. ≤20% TiO2 or ≤25% ZrO2+TiO2 can be used, if necessary, instead of said improver contg. ≤15% ZrO2.