PURPOSE: To inspect the leakage of a container automatically during the filling process of beverage, by optically measuring the size of the outer surface of a flexible container, which is filled with a beverage in a hot state and tightly sealed, and computing the value.

CONSTITUTION: A flexible container 101, which is filled with a beverage in a hot state and tightly sealed, is conveyed by a conveyer 102. Then laser distance meters 103a and 103b digitally measure the distance to the central part of the container 101. At the same time, the number of stepping X measured by an encoder 104 and the measured values Ya and Yb of the distance meters 103a and 103b are stored in a memory device 106. The difference Di between Ya and Yb is obtained by an adding device 107. The sum of the differences Di over the entire pulses (number of stepping) of the encoder 104 is obtained. The cross sectional area of the central part of the drum of the container 101 is compared with a preset value from a judging-level setting device 109 by a comparator 110. When the area exceeds the preset value, a defect signal is sent to excluding device. Thus the leakage of the container 101 is automatically inspected.