PURPOSE: To offer the response not causing waveform distortion while attenuating possibly ringing by specifying a time constant of a closed circuit comprising an externally mounted impedance and a restrictive capacitance of an output transducer.

CONSTITUTION: An ultrasonic wave solid-state propagation medium 31 is pro vided with an input transducer 34 converting an input electric signal into an ultrasonic wave and an output transducer 36 converting the ultrasonic wave into an output electric signal, and packing materials 49, 50 are adhered respec tively to the opposite face to the coupling face of the input transducer 34 and the output transducer 36 to the propagation medium 31. They are constituted by substantially equal acoustic impedances, and the externally mounted imped ance comprising at elast a resistor R is connected to the output transducer, and the time constant C0R of the closed circuit comprising the externally mounted impedance and the restrictive capacitor Co of the output transducer is selected as T0<RC0(2T0is a resonance period of the output transducer). Thus, a differential response with attenuated ringing without waveform distortion is obtained from the output transducer.