Silk fibers excavated at Fujinoki mounted tomb, Shimoikeyana mounted tomb and Kuriyama site were examined using FT-IR microscope and two peaks called amide I and II present in modern silk fibers were compared with those of excavated fibers. It was found that amide ground in a polymer molecule decreased with the progress of degradation and peak intensity of amide group in infrared spectrum decreased correspondingly. The advantages of FT-IR microscopy for the analysis of organic remains in archaeological view point were evaluated and it was concluded that the systematic investigation of spectral change due to the degradation of respective material is essential.