PURPOSE: To extract a reflected wave of an ultrasonic wave delay line and to obtain a delay time twice the delay time of the delay line by connecting an input of the ultrasonic wave delay line to an input side being an opposite phase to an output side of a buffer amplifier and extracting a signal from this connecting point.

CONSTITUTION: When a signal voltage of E1 is applied to an input terminal, the signal voltage E1 being an output of the buffer amplifier 5 and a signal voltage reflected at an output terminal of the ultrasonic wave delay line 4 and returned after 2τ appear at the input terminal of the ultrasonic wave delay line having a delay time of τ. Since the signal voltage E1 being the output of the buffer amplifier 5 is cancelled by a signal voltage -E1 being a differential input and opposite in the phase to that of the E1, the signal voltage delayed by 2τ is extracted with no adjustment independently of the input impedance of the ultrasonic wave delay line and the amount of reflection.