Characteristics of the absorption at 2170nm, which is due to peptide bonds, were investigated using various proteins. The intensities of the absorption due to peptide bonds were different depending on the kinds of proteins examined. The absorption of bovine serum albumin at 2170nm decreased by reducing its disulfide (S-S) bonds. The absorption of poly-L-glutamic acid in the a-helix state at 2170nm was stronger than that in the random coil state. These results suggest that the secondary structures of proteins affect the absorption at 2170nm. The extent of contribution of the secondary structures to the absorption intensity was statistically predicted using 9 proteins which have different contents in secondary structures such as α-helix, β-sheet and random coil structures. It was found that the relative extent of the α-helix, β-sheet and random coil structures to the absorption at 2170nm was approximately 2:1:1.