It is an object of the present invention to enhance reliability of an allowable limit conductance used for setting a threshold value for evaluating sealability of a test object, and further, enhance reliability of the sealability evaluation. By leak testing a test object 9A with a sealing defect 9g, a leakage characteristic that shows a relationship between an internal-external pressure difference and a leak flow rate is obtained. A conductance of a test object with a similar artificial leak 9B having a similar artificial leak device 29 disposed therein, conducting the leak-detected substance is measured. The similar artificial leak device 29 has a leakage characteristic similar to the obtained leakage characteristic. An allowable limit conductance is obtained based on results of the measurement. Sealability of a test object 9 is evaluated based on a threshold value set with a standard artificial leak device 9S having the allowable limit conductance.