Refracted angle for UT of axial groove type rotor is limited with the distance of the axial direction from the sensor installation surface. High intensity detection technique changes with the refracted angle, the change may require a combination of multiple inspection techniques. For this reason, detectable distance of a double probe technique and a tip echo technique are examined. In the distance more than 35mm, refracted angle become more than 35 degrees where refracted intensity is high and the defect is detectable using the double probe technique. In the distance of 35mm or less, signal to noise ratio of the tip echo technique becomes greater than two. Therefore combination of these UT techniques make possible to inspect overall distance without the dismantlement of rotor and blade.