Abstract: An islands-in-sea fiber includes island components and a sea component arranged to surround the island components, wherein the island components and the sea component are composed of at least two polymers and are provided on a fiber cross-section in a direction perpendicular to the fiber axis. The islands-in-sea fiber is a raw yarn for a combined filament yarn, which can be used to produce a cloth having good tension and drape and an excellent color-developing property. An islands-in-sea fiber in which at least two types of island components having different diameters are arranged on the same fiber cross section, the islands-in-sea fiber characterized in that at least one type of the island components have diameters of 10-1000 nm and have fluctuations in diameters of 1.0-20.0%.

Abstract: The fiber cross-section of an island-in-a-sea composite fiber perpendicular to the fiber axis, the island component and sea component are arranged such that the sea component surrounds the island components. The composite cross-section is very consistent, and the fiber has excellent post-processibility. The island-in-a-sea composite fiber wherein diameter of the island component is 10 to 1000 nm, variation of the island component diameter is 1.0 to 20.0%, modification ratio is 1.00 to 1.10, and variation of the modification ratio is 1.0 to 10.0%.

Abstract: A sea-island composite fiber has an island component which is ultrafine fibers having a noncircular cross-section, the ultrafine fibers being uniform in the degree of non-circularity and in the diameter of the circumscribed circle. The sea-island composite fiber includes an easily soluble polymer as the sea component and a sparingly soluble polymer as the island component, and the island component has a circumscribed-circle diameter of 10-1,000 nm, a dispersion in circumscribed-circle diameter of 1-20%, a degree of non-circularity of 1.2-5.0, and a dispersion in the degree of non-circularity of 1-10%.