To obtain a high-quality polyester fiber with a flat cross section which has a soft feeling, no feeling of greasiness being the defect of woven or knitted fabric composed of a conventional flat cross-section fiber, yet has transparency prevention, low air permeability, water absorption, quick-drying properties, abrasion resistance, improved color tone and a few fluffs.

The polyester fiber has a flat crosssectional shape of a single filament and is a flat crosssectional fiber in which the crosssectional shape of a single filament is a shape of 3-6 round crosssectional single filaments bonded in the length direction. The polyester constituting the fiber is a polyester obtained by polycondensation in the presence of a catalyst composed of a reaction product between a specific titanium compound and a phosphorus compound.