The cross-type differential sensor composed of sensor A and B can detect the orthogonal decomposition component of the leakage flux generated by the rotating field. The composition of these decomposition components clarifies the amplitude and the direction of leakage flux generated from the flaw. Figure shows the amplitude of the orthogonal decomposition component of the leakage flux vector and the composition vector by the scanning angle. The size of flaw can be evaluated according to the amplitude of a composition vector.