A calibration technique for gas-flow ionization chambers was studied for implementation of reliable radioactive gas monitoring. Three radioactive gases with short half-lives of 1 3 3 Xe, 1 3 5 Xe and 4 1 Ar were prepared by activating stable isotopes and used for the calibration. On the basis of activity determination by the DLPC method, a gas-flow ionization chamber used as a secondary standard was precisely calibrated in terms of ionization efficiency for each radionuclide. The influence of impurities in the 1 3 3 Xe gas on calibration of gas monitoring instruments is also discussed. This technique is considered to make the easy and reliable calibration of gas monitoring instruments possible.