PURPOSE: High-speed melt spinning is carried out under treatment with a specific jet fluid to produce extremely fine multifilaments that are free from filament fluffs and used as a base material for artificial leather.

CONSTITUTION: Thermoplastic polymer filament yarns that have a more than 100 filament count and filament fineness of 0.1W0.7 denier are produced by melt psinning at a speed of higher than 2,500m/min wherein the multifilament is treated with a jet fluid under conditions satisfying the equation to get a shape index of less than 7, which is defined as W/t where W is the width of the multifilament bundle and (t) is the thickness. The treatment with a jet fluid gives a multifilament with several hundred to several thousand interlaces per meter and reduces the flatness, resulting in reduction in filament fluff formation of the like.