Abstract: A distance between a distal end of a crack formed in a fatigue test piece on which a reference line is scribed, and the reference line, is monitored. The reference line and the crack are scanned with an ITV camera to prepare the fatigue test piece having the crack of a predetermined length. Luminance levels are integrated from a video signal obtained by scanning each raster in the same direction as that of the reference line to obtain a maximum luminance level, and the reference line position is detected from the maximum luminance level. At the same time, continuity of a high-level value is discriminated from the video signal, and a crack distal end position is detected from the discrimination result. A difference between both the detected positions is calculated, and the test piece is machined so that the difference corresponds to the predetermined length.