The present invention provides an islands-in-sea fiber which comprises island components and a sea component so arranged as 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 for producing 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 being 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%.
