A total injected electron quantity QBD, which has reached a constant value against a variation in stress voltage applied to an insulating film for use in a semiconductor device, is obtained as a critical injected electron quantity QBDcrit. The total injected electron quantity QBD is a total quantity of electrons injected into the insulating film before the film causes a dielectric breakdown. Thereafter, a time it should take for a total quantity of electrons, injected into the insulating film under actual operating conditions of the device, to reach the critical injected electron quantity QBDcrit is estimated as the expected lifetime of the insulating film.