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 133Xe, 135Xe and 41Ar 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 133Xe 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.