PURPOSE: To highly accurately detect flaw by detecting a flange position using a path line detection probe and controlling the vertical position of a flaw detection probe thereby compensating for the degradation of flaw detection accuracy due to fluctuation of path line.

CONSTITUTION: A path line detection probe 6 is secured to a position deviated from the welding position of the web 1W of a welding H-sections by means of a holding arm 21B secured to a post 20. A probe 6 is focused on the surface of the flange 1F of H-sections being carried continuously and the local point is detected in order to detect vertical fluctuation of the H-sections and thereby the fluctuation of the path line. When the fluctuation is within a threshold level, a flaw detection probe 2 is stopped at that position, otherwise, the probe 2 is shifted vertically by means of a holding arm 21A thus maintaining an appropriate distance between the surface of the flange 1F and the probe 2. This method lowers erroneous detection rate and enhances flaw detection coverage.