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 .mu.m or greater surrounds an alternating laminated section with a thickness of no greater than 10 .mu.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.