Abstract: A novel conjugate fiber with a fine and an optical interference color-generating function, suitable for application in product fields which require aesthetic qualities, the fiber being characterized by having a structure wherein an alkali-soluble polymer with a thickness of 2.0 ?m or greater surrounds an alternating laminated section with a thickness of no greater than 10 ?m, comprising alkali-insoluble polymer layers with different refractive indices alternately laminated parallel to the long axis direction of the flat cross-section, wherein the ratio (SP1/SP2) between the solubility parameter value of the higher refractive index polymer (SP1) and the solubility parameter value of the lower refractive index polymer (SP2) is in the range of 0.8-1.1.

Abstract: Polyester fibers having a deformed section are produced from a polyester polymer which is obtained by polycondensing an aromatic dicarboxylate ester in the presence of a catalyst containing a mixture of a Ti component (A) comprising at least one a titanium alkoxide or a reaction product thereof with a specific carboxylic acid or its anhydride with a P compound component (B) represented by the following general formula (III) and/or a reaction product of a Ti compound component (C) with a P compound component (D) represented by the following general formula (IV). The obtained fibers have a favorable color tone and excellent qualities without showing fluffing.