PURPOSE: When saturated polyester is melt-extruded, a solution of a powder additive in an organic solvent boiling at a specific temperature is fed quantitatively with a feeder to enable the uniform and quantitative feed of the additive for a long period of time.

CONSTITUTION: A powdery additive such as 2,2'-bis(2-oxazoline) is dissolved in an organic solvent boiling below 170°C; methylene chloride is preferred, when the additive is an iminoether, and the solution is fed to saturated polyester, preferably polyester mainly consisting of terephthalic acid and ethylene glycol, quantitatively with a solution feeder. Then, the saturated polyester containing the solution is extruded by heat melting into fiber. The concentration of the additive in the solution is preferably more than 1wt% and the solvent is evaporated with heat during melt- extrusion.