To provide a method for evaluating a corroded part, the method making it possible to specify only a waveform reflected by a corroded part and to evaluate the waveform. When a transmission unit (2) is moved on the surface of a metal pipe (60) and the distance between a corroded part (5) and the transmission unit (2) is changed, only a waveform portion A of ultrasonic waves reflected by the corroded part (5) moves toward the left or right along an X axis, and only the intensity of a noise waveform portion B included in a received wave changes upward or downward along a Y axis, which makes it possible to separate the waveform portion A and the noise waveform portion B of a longitudinal-wave surface wave reflected by the corroded part (5) and evaluate the waveform portion A in detail.