PURPOSE: To obtain the titled fibers, capable of removing water-soluble components with hot water and useful as ultrafine fibers, etc., by melt spinning a copolymer containing terephthalic acid as a principal component and 5-sodium sulfoisophthalate, polyalkylene glycol, etc., as one component, drawing the resultant fibers and heat- treating the drawn fibers.

CONSTITUTION: A copolyester containing terephthalic acid as a principal acid compo nent and 5-sodium sulfoisophthalate in an amount of 5W15mol% based on the total acid component, isophthalic acid and/or 3W20C aliphatic dicarboxylic acid in an am ount of 5W50mol% based on the total acid component and a polyacid anhydride from a 3W20C aliphatic dicarboxylic acid and/or polyalkylene glycol (derivative) having 400W6,000 average molecular weight and/or polyalkylene glycol (derivative) having 400W6,000 average molecular weight in an amount of 0W20wt.% based on the total polymer as copolymerization components is used as one component and subjected to melt conjugate spinning to provide fibers, which are drawn and heat-treated at a temperature of ≥20°C lower than the softening temperature [(Ts)°C] of the copoly mer without causing mutual melt sticking of the conjugate fibers to afford the aimed conjugate fibers.