The invention concerns a gas leak detector (1) having an inlet (3) enabling connection to a test piece or test chamber, a detector unit disposed within the detector (4), a high-vacuum pump (5) which is connected at one end to the detector unit and through which a test gas, such as helium, passes in the opposite direction, a back-up pump (6), a line (9, 13) joining an outlet of the high-vacuum pump (5) with an inlet of the back-up pump (6), and a line (7, 9) joining the test-piece connector (3) to the high vacuum pump outlet. In order to reduce the complexity and expense necessary to produce the back-up vacuum, the invention proposes that line (13) is fitted with a throttle (14) and including a further line (15) which is fitted with a valve (16), said line directly joining the test piece (3) to the inlet of the back-up pump (6).