PURPOSE: To achieve an automatic correction of a flaw detection signal confirming the reliability of a flaw detection signal by arranging ultrasonic wave transmitting the receiving elements so as to face each other at a fixed angle with a coupling check ultrasonic wave as transverse wave.

CONSTITUTION: The mounting position of a coupling check ultrasonic wave transmission element 14 and a coupling check ultrasonic wave receiving element 15 coincides with the incident point 5 longitudinally while sandwitching flaw detection ultrasonic wave transmitting/receiving element 2. They are so inclined that a coupling check ultrasonic wave 16 transmitted from the coupling check ultrasonic transmitting element changes to a transverse wave in the vibration mode while being reflected from the bottom surface of a test material 4 to be returned to the coupling check ultrasonic wave receiving element positively when entering the test material 4. In this manner, the vibration mode of the coupling check ultrasonic wave in the test material can be the same in the vibration mode (transverse wave) as when a flaw detection sound wave propagates to confirm the reliability of a flow detection output signal thereby enabling automatic correction of flaw detection signals.