PURPOSE: To obtain a carbon-isotpe analyzing apparatus which can trace the isotope ratio of carbon accurately in high sensitivity.

CONSTITUTION: A semiconductor laser 10 in a near-infrared-ray region, a means 11 for sweeping the oscillated wavelengths of the semiconductor laser 10, a frequency modulating means 13 for applying the frequency modulation on the semiconductor laser 10, a photodetector 16 for detecting the laser light which has passed through a specimen wherein a plurality of the carbon isotopes are mixed and a lock-in amplifier 19 for detecting the matching of the modulating frequency of the modulating means and the signal frequency of the laser light detected with the photodetector are provided. The ratio between the intensities of the light absorbing spectrum of 12CO2 when the light-emitting wavelength of the semiconductor laser 10 is the wave number 6350.15±0.2cm-1 and the light absorbing spectrum of 13CO2 when the light-emitting wavelength is the wave number 6350.36±0.2cm-1 is detected.