PURPOSE: To manufacture polyester multi-filament having excellent uniformity, in high efficiency, by melt-spinning a polyester, and cooling the spun filament with cooling air stream having a relatively high temperature and blasted perpendicularly to the filament from one side.

CONSTITUTION: Spun polyester filaments obtained by the melt-spinning of a polyester are cooled by using a single-side blasting device. The cooling air stream at the upper part of the cooling device 1 passes slightly downward through the running multi-filament 3, and the temperature of the cooling air is adjusted to (T+5)W50°C (T is ambient temperature).