A helium supply unit (1) supplies helium from a high pressure storage (21) to fill a test chamber (6) which encloses a sample holder (5) to be examined for leaks by measuring device (2) including a mass spectrometer (3) and a control unit (4). The test gas is recovered after the testing by a pressure difference between a low pressure storage (18) and the test chamber (6). The test chamber is evacuated prior to receipt of the test gas by a vacuum pump (27) and after testing by a vacuum pump (34) for more complete helium recovery. The recovered test gas is then transmitted back to the high pressure storage (21) through a compressor (19), which raises the pressure of the recovered gas to a predetermined pressure, from the low pressure storage (18). The control unit (4) also controls a pure test gas supply (37) and a release valve (44). The concentration of the recovered test gas is measured and, if the concentration is too low, the control unit (4) releases the recovered test gas through valve (44) and admits fresh test gas from the supply (37) into the helium supply unit (1) at a point between the low pressure storage (18) and the compressor (19). In this manner, expensive test gases, such as helium, are recovered and reused.