A method to monitor in situ the dose rate from the gaseous radionuclide 41 Ar is developed using a portable gamma-ray spectrometer. A high-purity germanium detector with a sensitivity of 0.358 nSv/h per count per minute is used to calibrate 1294-keV gamma rays emitted from radioactive 41 Ar. Field measurements are conducted both inside and outside of the containment of a nuclear reactor during full-power operation, and iso-dose rate contour curves are mapped. The in situ measurement can be readily performed at various locations near a nuclear reactor with a 14-kg portable spectrometric unit. The detection limit for a 1-h counting period is as low as 0.35 nSv/h for the gaseous 41 Ar. One can use the method and field measurements developed in this research to quantitatively determine the gaseous fission products of krypton and xenon dispersed from a nuclear power plant.