The present invention relates to a functional fiber for preventing forgery, more particularly functional fiber for preventing forgery which exerts functionality by mixing special pigments to compositefibers of side by side shape on which two components are attached. The functional fibers of two components structure according to the present invention are consisted of synthetic high molecules, andhave functionality which fibers display color by colored pigments with visual or fluorescence by light of ultraviolet ray or absorbing property of infrared pigments by being produced by mixing coloredpigments, fluorescent pigments, infrared pigments and the like to each resin, and then melt-spinning the mixture, or post treating it. While such fibers according to the present invention are hard tohave a property of fibers since those contain pigments above a specific concentration, the fibers have functionality for preventing forgery by mixing a monofilament which is obtained by cutting the fibers to papers, resins of high molecules, or coating agents so that it can be used as a functional element for preventing forgery and identifying authenticity of particular product demanded securitysuch as a bank bill, a bond, a passport, an identity card.