To always correctly compensate drift by estimating a drift compensation quantity of a leak inspection device from a drift characteristic curve.

After estimating a value of differential pressure caused between an inspection body and a reference tank as a first measured value D1, the variation of the differential pressure caused during elapsing of a specific time from a time point having passed longer time than the specific time is obtained as a second measured value ΔD3. Gaining the first measured value and the second measured value by using the inspection body set at a different temperature from a reference ambient temperature and a seal jig maintained at nearly equal temperature to the reference ambient temperature is executed at every plurality of temperature difference by changing the temperature of the inspection body. The first characteristic curve X1 of each temperature difference vs the first measured value D1, and the second characteristic curve X2 of each temperature difference vs the first measured value ΔD3 are recorded. The temperature characteristics of the drift value included in the first characteristic curve X1 is obtained from the difference between the first characteristic curve X1 and the second characteristic curve X2. By utilizing the temperature characteristics of the drift value in the inspection mode, the drift compensation value is obtained.