PURPOSE: To achieve a flaw detection accuracy, by arranging an area between an acoustic lens and a piezoelectric element to give a high impedance with respect to an ultrasonic wave within a longitudinal wave critical angle among ultrasonic wave outputs from the acoustic lens to prevent the mixing of longitudinal and a transverse waves.

CONSTITUTION: An acoustic lens 8 has a recess composing an air layer 9. The air layer 9 cuts a contact of a piezoelectric element 2 with the acoustic lens 8 so that an ultrasonic wave from the piezoelectric element 2 passing through the air layer 9 from will not reach an object to be inspected. An ultrasonic wave incident into a sample above a certain angle α1 (critical angle) will generate no longitudinal wave. As a transverse wave also has an critical angle α2 (α1<α2), any ultrasonic wave which is incident into the object to be inspected beyond the α2 is totally reflected. Therefore, the area is so formed that an ultrasonic wave passing through the air layer 9 is such as to be focused within the critical angle α1 to confine the ultrasonic wave generated in the object being inspected solely to the transverse wave thereby achieving a highly accurate flaw detection.