PURPOSE: To output only a defect signal by removing a waveform caused by an external factor other than a defect on a metal surface from a detected signal.

CONSTITUTION: An inspecting signal A of a detector outputs a signal X0 through an HPF15, an inspecting signal component Y0 for a direction phase shifted by 90° against a phase of the signal X0 in a signal A is obtained. There is mixedly the waveform due to the external factor in addition to the waveform causes by an external factor in the signal X0. Thereupon, by taking a logical product by adding the signals X0, Y0 to a gate 19, the waveform due to the external factor is eliminated, and only the flaw signal X1 is obtained. The signal A forms a lift-off signal B through an LPF20, adds the signal B to a gate 21 to take a logical product with the signal Y0, and a lift-off correcting signal C is obtained. Next, by processing the signals C and X1 by a correcting circuit, an erroneous signal due to the external factor and the influence due to the lift-off variation are removed from the output signal, and only the flaw signal corresponding to the depth and length of the actual defect is obtained.