PURPOSE: To use even under severe environment by allowing a liquid flow to collide against a body to be inspected, and transducing the impulsive sound or the impact pressure generated at that time to an electric signal.

CONSTITUTION: In case when a steel sheet 10 is not at a position of a water flow W jetted from a nozzle 22, the flow W gives an impact to a diaphragm of a receiver 24. The impulsive sound at this time reaches a microphone 28 through a tube 26, is transduced to the electric signal and is then inputted to a signal processor 30. The processor 30 consists of a band-pass filter 30a, an amplifier 30b and a comparator 30c. In case when the plate 10 comes to the position of the flow W, the flow W doesn't collide to the diaphragm of the receiver 24, and the detecting signal of the processor 30 is turned on.