[en] The measuring methods for tritium and carbon-14 and their results over seven and twenty years respectively in National Institute of Radiological Sciences are reported. For measuring C-14, the method of obtaining methane from carbon dioxide by adding hydrogen and heating at 480 degree C in the presence of Ru catalyst was published, and afterwards, it was found that when the above reaction is proceeded with the pressure of 10 kg/cm², the reaction temperature can be lowered to approximately 280 degree C. In addition, C-14 in refined vegetable oil and alcohol, and the annual change of C-14 concentration in environment have been measured. The tritium concentration in river water and coastal sea water all over the country including the areas around nuclear facilities has been measured since 1967 and its tendency has been grasped. Discussion on its detectable limit by liquid scintillation spectrometer method leads to the development of a low background liquid scintillation counter. The above tritium concentration differs remarkably between river water and coastal sea water, the former being two to five times as much as the latter. As the concentration decreases gradually every year, the major part of present tritium is the fall-out due to nuclear tests. (Wakatsuki, Y.)