PURPOSE: To precisely and simply detect the presence of cracks in a ceramics to be tested and crack positions, by scanning the surface of the ceramics to be tested with a thinly focused ultrasonic beam to detect electric oscillation corresponding to piezo-electric oscillation.

CONSTITUTION: In a crack test for a large area and thin type piezo-electric ceramics 1 such as a piezo-electric speaker and a piezo-electric microphone, an electrode 2 consisting of Ag, Ni, etc. is formed and a metallic disc 3 made of brass e.g. is mounted on the speaker e.g. The metallic disc 3 is mounted on a conductive board 5 and an ultrasonic scanner 6 is arranged on the side of the electrode 2 to amplify 20 a high frequency signal from an oscillator 19, to supply the amplified signal to a piezo-electric element 9 in the scanner 6 and generate an ultrasonic signal. The ultrasonic signal is transmitted to a lens by an ultrasonic wave propagating element 10 to focus an ultrasonic beam and to make the beam incident to the ceramics 1 through water. If a crack 4 exists, oscillation of the ceramics 1 is reduced and low-voltage signals 24 are generated. These signals are displayed on a oscilloscope 14 or an alarm is generated by an alarm driving circuit 18.