PURPOSE: To prevent plural sonars from interfering with each other and to enable target detection by varying a transmission frequency and a reception frequency band corresponding to the travel azimuth of the active sonar device.
CONSTITUTION: The travel direction of the active sonar device (AS) is detected by an azimuth detecting circuit 8, whose output is sent as an azimuth signal 9 to a sent signal generator 2 and a band-pass filter 5. Then the generator 2 varies the frequency of an electric signal for transmission with the signal 9 and the filter 5 varies the passing frequency band of an electric signal for reception. At this time, a sound wave of the transmission frequency is sent 1 into water and the reception frequency band is matched with the underwater sound signal frequency band wherein a receiver 6 receives the sound wave. Consequently, when, for example, two underwater sailing bodies 11 and 12 send directional beams 11' and 12' of transmission frequencies f 2 and f 4 while sailing opposite each other, the reception bands of the AS are different and their received sound waves are outside the reception parts of the opposite waves and, therefore, never interfere with each other, so that the target is detected.