In order to judge the acceptability of internal pressure of a can accurately based on the displacement of the can, a pair of displacement sensors (2, 3) facing each other coaxially are arranged perpendicularly to the carrying direction of a carton case (C) such that the center of an encased can (K) being carried matches the center of the pair of displacement sensors (2, 3), measurement data of the displacement sensors (2, 3) is taken in at the timing that the center of the can (K) passes through the center of the pair of displacement sensors (2, 3), a total depth (Ds=Dt+Db) of a top depth (Dt) from the upper end of the can (K) to the open tab and a bottom depth (Db) from the lower end of the can (K) to the bottom panel is calculated, and then the calculation result is applied to a criterion for judging the acceptability of the total depth corresponding to the internal pressure of acceptable can.