To provide an air leak test method capable of performing highly accurate tests in a short time.

In normal test mode, an initial pressure from a pressure supply means 10 is supplied for a master-side branch passage 21 and work-side branch passage 22 via a common passage 20. Then after a test pressure is supplied, opening/closing valves 27 and 28 are closed and waited for a predetermined waiting time, a leak from a work W connected to the work-side branch passage 22 is detected from a differential pressure detected at a differential pressure sensor 29. Furthermore, in special test mode to be executed as necessary or periodically, after the opening/closing valves 27 and 28 are closed and waited for a longer waiting time than in the normal test mode, a differential pressure component which linearly changes due to leak is subtracted from the detected differential pressure to obtain a differential pressure component which does not change. The time from the start of the initial pressure supply to the switching to the test pressure, a pressure difference between the initial pressure and test pressure, etc., are adjusted so as to reduce the differential pressure component which does not change to zero.