PURPOSE: To detect an axial defect in short time, by generating an SH wave propagating spirally through a tube from one side to receive a lamb wave propagating to the other side and generated when the SH wave is projected slantly into a defect present along the axis of the tube.

CONSTITUTION: Magnet belts 20a to 20f of S and N poles are arranged spirally on the side of a cylindrical magnet 20 with a core 21 inserted thereinto at a pitch equivalent to 1/2 of the wavelength λ of an SH wave and when an SH wave is transmitted to the magnet belts along the axis of a tube 10 from a transmitting ultrasonic transducer (EMAT) 12 so arranged to be wound with a coil 22, the SH wave propagates spirally and is projected slantly (at about 35°) into a defect 11 generated along the axis of the tube 10. With such an arrangement, the SH wave is transformed into a Lamb wave propagating axially by a mode conversion and travels in the direction opposite to the side where the transmitting EMAT12. Then, the Lamb wave is received by a receiving EMAT13, the same in the structure as the transmitting EMAT12, arranged in the ongoing direction of the Lamb wave to detect an axial defect 11.