Infrared spectra of substituted pyridine 1-oxides with various kinds of substituent mainly at 3- or 4-position were determined. Their N-O stretching frequencies (strong absorption in the region of 1200 and 1300cm^-1) were found to be correlated with the nature of a substituent, and a linear relation between these frequencies and σ-values of the substituents was pointed out. The ring C-H out-of-plane bending frequencies of these compounds showed a regular shift from those of the parent pyridines, shifting always to a higher frequency in 2- and 4-substituted derivatives, and always to a lower frequency in 3-substituted derivatives. Absorptions in the regions of 900∼1200cm^-1 and 1450∼1630cm^-1 were also correlated with the structure. From a comparison of the frequency shift in a characteristic vibration of the ring substituent, the σ-values of 0.25 and 1.18 were respectively obtained for 4- and 3-positions of pyridine 1-oxides. All these points were discussed in terms of an electronic effect of the N-oxide function.