The ultrasonic-wave wavemeter, the so-called inverted echosounder, which the authors developed in 1969, has been used as a standard wavemeter for the coastal wave observation in Japan. However, there sometimes appeared abnormal records in measured wave profiles. Upon investigation of the causes of the abnormal records which has been carried out at Sakata Port and Hajikizaki, it was found that they had been caused by air bubbles due to wave breaking and floating matters (for example littoral drift etc.). Because of them, the surface echo was much attenuated and reflected abnormally. The fluctuating width of the surface echo S was about 80dB and the ratio of the surface echo S to the noise echo N(S/N) was less than the unity.
　In order to remove the noise, we have proposed a new method, in which the surface echo S is detected by the backward edge, instead of the forward edge, of the signal pulse. So we have newly developed two circuits named APC and ATL to realize the method. The former automatically modulates the oscillated sonic power and the latter automatically sets the trigger level, corresponding to the input signal.
　The results of the application of the circuits to the field observations indicate that the above-mentioned defects in the traditional ultrasonic-wave wavemeter can be avoided to the extent of not less than 90%.