PURPOSE: To continuously tap cans with a signal corresponding to vibrating sound by a method wherein the vibrating sound of a container wall is given to a can-tapping unit as feedback.

CONSTITUTION: When a can 11 passes through an inspecting position 13, a pulse current is supplied to an electromagnetic coil 15 and a tap is given to the can. The sound produced by tapping is sent to a logic circuit 25 and then to a switch 22 from a microphone 18 through an amplifier 24. After a tap signal has been sent, the output of a time setter 26 works to change over the switch 22. The signal is stored in a can tapping power amplifier 23 as feedback, which supplies AC current to the electromagnetic coil 15. Due to this, the can 11 is again tapped, while an oscillating loop is formed with the can as a vibrator. For this reason, the increased vibration in the can wall prevents the occurrence of the parasitic and abnormal oscillations of the can.