PURPOSE: To simultaneously form further many received sonic lines so as to reduce the number of parts, therefore, to reduce power consumption, by converting the signal received by a vibrator element to a light signal and delaying said light signal by a optical fiber to convert the same to an electric signal to form sonic lines.

CONSTITUTION: The ultrasonic signal received by a vibrator element IL0 is amplified by a preamplifier A0 and the output signal thereof allows a light emitting diode LED0 on an element side to emit light which is, in turn, transmitted to photodiodes PD by optical fibers OF0-OF127. In the same way, the received signal of an element EL63 allows a light emitting diode LED63 to emit light which is, in turn, transmitted to a photodiode PD0 on an output side by an optical fiber 63F0. The length of each of the optical fibers OF0-63F0 is controlled in order to delay a light signal so as to form one sonic line and the output signal of an amplifier A0' forms the 0-th sonic line and, in the same way, the output signal of an amplifier A127' forms the 127-th sonic line. By this method, the optical fibers F connecting the light emitting photodiodes LED and the photodiodes PD constitute a perfect group formation.