A method for manufacturing an optical fiber includes: heating an optical fiber preform to draw glass fiber; measuring an outer diameter of the glass fiber to obtain a function of time; transforming the function of time into a function of frequency; identifying a first peak caused by a first drawing condition and a second peak caused by a second drawing condition in the function of frequency; and adjusting the second drawing condition so as to satisfy fn<fm−wm/2 or fn>fm+wm/2, where fm is a frequency of the first peak, wm is a full width at half maximum of the first peak, and fn is a frequency of the second peak.

Provided are flat-cross-section glass fiber that is superior in continuous-production efficiency and mass-production suitability and able to give superior reinforcing ability for resin molded products, and a method for manufacturing flat-cross-section glass fiber from a glass raw material containing a recycled glass material. In the flat-cross-section glass fiber of the present invention, flat-cross-section glass filaments constituting the flat-cross-section glass fiber each have a flat cross-sectional shape having a major axis with a length in a specific range and a minor axis with a length in a specific range, and the coefficient of variation for the lengths of the minor axes is within the range of 8.0 to 26.1%.