To make it possible to simply measure the internal pressure of a vessel at a low cost by the previously obtained relationship between the resonance frequency and the internal pressure of the vessel by exciting the vessel and measuring the resonance frequency.

The metal drum, to which a top plate 2 and a base plate 3 are fixed at a step 1, is heated at a step 2, then the outer surface is coated, and heated at 65°C for 15min in a drying furnace. Then, dehumidified air is introduced into the vessel, and a large plug 4 and a small plug 5 are closed. Thereafter, it is naturally cooled to the ambient temperature at a step 3. After cooling, the pressure in the drum becomes lower than the atmospheric pressure. The pressure change in the drum is measured by a vacuum pressure gage. The internal pressure of the drum which cannot hold the airtightness is near the atmospheric pressure as compared with the drum which can hold the airtightness. Since the resonance frequency generated by hitting the cover of the vessel and the internal pressure in the drum have a predetermined correlation, the airtighness of the drum can be inspected.