PURPOSE: To surely and quickly detect defect of airtightness of a flexible sealed container in advance.

CONSTITUTION: A container 30 is transferred while being pressed and pinched by a belt unit consisting of (the upper part) of a lower belt 5 and (the lower parts) of first and second respective upper belts 1, 2. A force-detecting unit 10 with a force sensor 13 and roller for pressing of the container is provided at a space between the final ends and start ends of respective upper belts 1, 2, and the force-detecting unit 10 detects a defective airtightness of the container 30. A position sensor 31 assigns a fetching timing for the output of the force sensor 13. A judging unit 6 determines an abnormality when the fetched output of the force sensor is at a set value or less. This judgement signal is displayed on a display apparatus 7, and also operates a discharge gate 8, and the container 30 is discharged. When the output is normal, the discharge gate 8 is positioned as shown by a solid line, and the container 30 is transferred to the next process by a transfer belt 9.