In decommissioning process of nuclear facilities, large amount of radioactive isotopes are discharged as waste. Radioactive carbon isotope (¹⁴C) is one of the key nuclides to determine the upper limit of concentration in the waste disposal. In particular, ¹⁴C on the graphite reactor decommissioning should be separated from stable carbon isotopes (¹²C and ¹³C) and monitored for the public health and safety. We propose an isotope analysis system based on cavity ring-down laser spectroscopy (CRDS) to monitor the carbon isotopes (¹²C, ¹³C and ¹⁴C) in the isotope separation process for the graphite reactor decommissioning. This system is compact and suitable for a continuous monitoring, because the concentration of molecules including the carbon isotope is derived from its photo absorbance with ultra high sensitive laser absorption spectroscopy. Here are presented the necessary conditions of CRDS system for ¹⁴C isotope analysis through the preliminary experimental results of ¹³C isotope analysis with a prototype system.