Ultrasonic sensors which use copolymer membranes, just like other single-crystal elements and ceramic elements, are able to transmit and receive with the same element. However, for meeting the required specifications to identify welding defects around 0.1 mm, there were many difficulties and problems in manufacturing an integrated type of sensor with functions for both transmitting and receiving, as well as from a costwise consideration. Consequently, to overcome these problems, the following new concepts were introduced for quick implementation. As far as the thin copolymer membrane is concerned, the reception part and the transmission part were not separated but integrated, and an independent electrode was installed for each function. The beam-receiving surface of the reception part of the membrane was made so as to form an overall earth electrode, while on the opposite surface, a number of comblike electrodes were installed in parallel. Furthermore, because the electrical impedance of the piezoelectric copolymer sensor is low, careful attention should be paid to matching between the cable and the electrical circuit. With this device, the installation of a preamplifier adjacent to the receiving element solved the problem. The developed ultrasonic sensor has a 1-channel transmitter with a frequency of 20 MHz and high-density receivers with 56 channels on a 20-mm-long membrane.