PURPOSE: To eliminate a region not yet subjected to flaw detection between flaw detection scanning loci and said region in the scanning advance direction by performing the flaw detection of the overall surface with respect to a steel plate by twice flaw detection scannings.

CONSTITUTION: A probe group having a flaw detection efficient width W is provided to a shoe 6 attached to a probe holding mechanism having a size of width X2. The shoe is attached to the flaw detecting mechanism apparatus at intervals of the size 2W. The shoe 6 moves relatively to the arrow direction (a) from the left end of the steel plate 7 to be tested and stops at the right end of the plate. The shaded part is the locus of the flaw detection scanning at this time. Next, the probe group stopped at the right end of the plate moves all together by a distance W in the arrow direction (b) by a flaw detection truck. At this time, the shoe 6 moves all together to the arrow direction (b), fills the brank space part by the second flaw detection scanning, and stops when it arrives again at the left end of the plate.