To obtain a can hitting inspection system by which a proper can be sorted by a method wherein the natural oscillation frequency of a can to which a shock is given and the displacement of the bottom of the can are detected, the positive pressure and the negative pressure of a pressure inside the can are judged on the basis of a displacement amount, the pressure inside the can is found on the basis of the natural oscillation frequency and an improper can be removed.

A can 2 to be inspected is conveyed on a conveyance belt 1, and its arrival is detected by a light source 3-1 and a light receiving element 3-2 in an existence detector 3. The distance between the ring-shaped grounding face and the bottom-wall central part of the can 2 to be inspected is found by a displacement meter 4, it is compared with a reference value in a RAM inside a microcomputer 10, and a positive-pressure region is discriminated from a negative-pressure region. A shock current is made to flow to an exciter coil in a hitting inspection probe 5, and a vibration sound is detected by a microphone. A high-frequency region and a low-frequency region which are outside an estimation vibration region are removed from the vibration sound by a frequency analysis part 11. After that, the natural oscillation frequency of the can 2 is extracted from its histogram so as to be sent to the microcomputer 10. Therefore, by using information on a characteristic curve, between the pressure inside the can 2 and the natural oscillation frequency, which is stored in the RAM or the like, a proper pressure inside the can 2 can be judged.