Abstract: Provided is a polyester binder fiber which contributes to produce a liber structure having a high strength. The polyester binder fiber may have a ?H of 30 J/g or less which is calculated as a difference between a melting endothermic amount ?Hm and a crystallization exothermic amount ?Hc in a temperature elevation phase as recorded by differential scanning calorimetry (DSC) curve. The polyester binder fiber may have a ratio (a1:(a2+a3)) between a crystalline component fraction (a1) and a total amount of a constrained amorphous component fraction (a2) and an amorphous component fraction (a3) in the range of from 98.0:2.0 to 50.0:50.0 in which the crystalline component fraction (a1), the constrained amorphous component fraction (a2), and the amorphous component fraction (a3) are calculated from a spin-spin relaxation time T2 at 140° C. obtained by pulsed nuclear magnetic resonance (NMR) spectroscopy.

Abstract: Provided is a sea-island composite fiber having excellent spinnability and thermoplastic deformability. The sea-island composite fiber comprises an island component and a sea component having a lower melting point than that of the island component, wherein the fiber has an aspect ratio of from 2.0 to 5.0 in a fiber cross section; and the sea component has a sea component thickness of from 0.2 to 2.0 ?m, the sea component thickness being defined as a distance between an outer periphery of the fiber and an island component closest to the outer periphery on a minor axis of the fiber cross section.