Abstract: A gas leakage testing method to be applied to a gas leakage testing system is provided to effectively detect whether or not any leakage points exist in a testing specimen. In a gas leakage test, the testing specimen is stored in a testing container in an air tight manner, and then, a leakage detector searches the leakage point in the testing specimen by use of probe gas (i.e., helium gas). However, due to the existence of the probe gas component in the atmospheric air, a background noise occurs, and it deteriorates the detection precision of the leakage detector. In order to avoid such drawback, a vacuum evacuation is carried out so as to reduce an internal pressure of the testing container to a predetermined low pressure (preferably, ranging from 500 hPa to 1 hPa), and then, the testing container is purged with substitute gas (e.g., nitrogen gas).