Glass fiber production method

A method for producing glass fiber, when glass fibers are formed by discharging, from a nozzle tip, a molten glass obtained by melting glass raw materials mixed so as to give a glass composition comprising, when melted, in relation to the total amount thereof, SiO2 in a range from 57.0 to 58.8% by mass, Al2O3 in a range from 18.0 to 20.0% by mass, MgO in a range from 7.5 to 12.0% by mass, and CaO in a range from 9.0 to 12.5% by mass, and having a total content of SiO2, Al2O3, MgO and CaO of 98.0% by mass or more, the glass composition comprises B2O3, or a combination of B2O3 and Li2O, as an additive or additives capable of suppressing generation of red crystals,

• wherein a ratio of the product between a content of the additive or additives capable of suppressing generation of red crystals (% by mass) and the content (% by mass) of CaO to the content (% by mass) of Al2O3 is in a range from 0.25 to 1.00.