Measurement of carbon concentration in CZ silicon by infrared absorption spectroscopy was examined. Noise level was suppressed down to 10-4 in unit of absorbance. Residual differential absorption between the sample and reference was removed by fitting the phonon absorption spectrum to the background absorption spectrum. The effect of narrowing of absorption spectral range was examined. As a result, it was possible to measure the differential carbon concentration down to about 1×1014/cm3. Measurement of commercial wafer was also established.