PURPOSE: To calculate easily the volume of gaseous body in an air-containing hermetic package that contains contents and the gaseous body by a method wherein the package is cooled down or heated up and after that, an area at the temperature in a specified range of values on the surface of the package is detected.

CONSTITUTION: An air-containing hermetic package A obtained by packing contents such as curry into a retort pouch together with gaseous body is conveyed, being laid down on a conveyer 101, and cold air is blown during the conveyance onto the package by a cold-air-supplying device 103, and thereby temperature difference is made discernible on the package A between a part contacting the gaseous body and a part contacting the contents. Then, the temperature over the surface of the package A is detected by a detector 211 utilizing infrared rays, and the obtained temperature information is converted into thermal image data by means of a conversion circuit 20, and is stored in a storage section 30. The area at the temperature in a specified range of values on the surface of the package A is detected from the thermal image data in the storage section 30 by a detecting means 40, and the volume of the gaseous body in the package A is calculated by a calculation circuit 50 out of the detected area, and is displayed on a display unit 60.