PURPOSE: To enable transmission abnormality to be identified without fail and to eliminate the errors in time measurement by reading the gate pulses which follow received ultrasonic wave pulses and the output of a D-FF to which the detected received wave pulses are inputted by a next D-FF immediately before the reset.

CONSTITUTION: The gate pulses which follow the received ultrasonic wave pulses are applied to a D terminal of the D-FF18, the detected received signal wave pulses Z are applied to a T terminal, and a reset signal R, which is generated before estimated time point when next pulses Z are generated, is applied to a reset terminal C. An output signal F' is applied to a D terminal of the D-FF19, and a sampling input signal S, which is generated after the time point when the pulses Z are estimated to be generated and before the reset signal R is generated, is applied to a T terminal. Either an output signa 1 or a 0 from the FF19 is related to the determination for designating the received ultrasonic wave as a normal wave. Even though the receiving wave detection occurs during the time the gate pulses are not generated due to te amplitude variation and even though the detection cannot be made due to the disapperance of the receiving wave, the identification of abnomality is possible and the errors in time measurement can be eliminated.