[en] We developed a nondestructive measurement method of defect shapes (single crack or branched crack and inclination angle) existing on the surface of steel flat plate of uniform thickness by ultrasonic waves incident from the back surface. The change in reflection intensity (C scan image) due to the probe position was measured by the normal beam technique by single transducer operation with immersion testing. In this study, the defect shape is evaluated from the characteristics of interference fringes of reflected intensity. Asymmetrically branched defects and symmetrically branched defects are identified using the deviation from symmetry of interference fringes of reflection intensity and, the change rate of reflection intensity with probe position, indicating their effectiveness. (author)