Oblique angle flaw detection method and device

The present invention provides a method of and a device for oblique angle-detection for detecting a flaw in welded portions in a nuclear power plant. The oblique angle detection method comprises a step of setting a delay time corresponding to a relative position of an ultrasonic probe and an objective member and a step of adding waveforms of the received signals displaced by the predetermined delay time. Then, the intensity of the reflected waves is enhanced by the addition of the waveforms of the received signals to detect the presence or absence of the objective flaw. The adding step comprises (a) determining an assumed position of the flaw in the x direction of the measured position based on the peak value of the signals prepared by adding received signals in the y direction, and (2) setting a time shift amount corresponding to a relative position in the x direction based on the assumed position of the flaw and adding the waveforms of received signals. (I.S.)