PURPOSE: To decide whether or not the internal pressure in a sealed container is proper or not without reference to the positioning accuracy of the container by measuring the quantity of stress nearby the center part of the lid part of the container on the basis of the peripheral part of the lid part and deciding whether or not the internal pressure is proper or not from the measured quantity of the stress.

CONSTITUTION: An abutting part 4 is made to abut on the peripheral part of the lid 2 of a bottle whose internal pressure is normal to position a frame 5 at the peripheral part 2b of the lid 2, and a knob 9 is rotated to set the distance D between a distance measuring means 6 and the abutting surface 4a of the abutting part 4 properly. Many normal bottles are measured similarly to obtain a range of variance. The abutting part 4 is made to abut on the peripheral part of the lid 2 of a bottle to be measured and the distance to the center part of the lid 2 is measured by the distance measuring means 6. Then this measurement result is compared with the previously measured values of the normal bottles and it is decided that its internal pressure is normal when the result is within the range of variance.