Handling of a large amount of tritium and tritiated contaminants had been carried out many times repeatedly in the OKTAVIAN facility which is an accelerator of Cockcroft Walton type to produce 14 MeV fast neutrons by D-T reaction. To estimate the dose due to internal exposure following intake of tritium, the distribution of tritium concentration has been measured with the bioassay method and the liquid scintillation counting method by using bioassay samples in man such as urine, exhaled water and so on. On the basis of their many tritium concentration data accumulated in the OKTAVIAN facility until now, a new tritium metabolic model has been developed by modifying a conventional three-compartment model known as the most famous model. The present model was verified using measured data, and compared with other models proposed previously.