PURPOSE: To obtain a ZnO single crystal having electric conductivity in one single crystal which can deal with a wide frequency range used for ultrasonic inspection of various materials to be measured by forming such a ZnO single crystal that shows variation of electric conductivity in the a-axis or c-axis direction.

CONSTITUTION: The ZnO single crystal shows variation of electric conductivity in the a-axis or c-axis direction. Even when ultrasonic inspection is performed by using frequency in the frequency range (10kHz-50MHz) usually used for ultrasonic inspection, the ultrasonic wave of this frequency passes through the ZnO single crystal area showing proper acoustoelectric exchange characteristics for this frequency when the ultrasonic wave of this frequency passes through the ZnO single crystal. By this method, sufficient detection sensitivity can be obtd. for a wide frequency range.