PURPOSE: To generate an ultrasonic wave over the overall circumference of a thin pipe efficiently without rotating the thin pipe, etc., by generating a Lamb wave having a vertical transversal wave component in the thin pipe.

CONSTITUTION: Cylindrical supporters 12a and 12b are provided with ferrite core 13 and permanent magnets 14, having the polarities uniformly, by turns, and a coil 15 is wound around them to constitute an EMAT11. In this case, t0=T0/4. The EMAT11 is inserted into a thin pipe 4 to be checked and a high-frequency current is flowed through the coil 15 to generate an eddy current I parallel to the connection direction of the thin pipe 4 in the pipe 4 by electromagnetic induction. Then, a Lorentz force F is generated through interaction of the eddy current I and magnetic flux B1 and B2 in the thin pipe. This Lorentz force F varying in direction at the same cycles with the cycles T0 of the magnetic flux distribution. As a result, a Lamp wave having a transversal wave perpendicular to the internal surface of the thin pipe 4 is generated by the force F at the circumference of the pipe 4 to propagates in the thin pipe 4, and is reflected at a defective, etc., of the pipe 4 to return. This returning wave is detected.