To provide a plate wave ultrasonic flaw detection method enabling the use of a small capacity memory to lower apparatus cost.

An image forming part 62 successively arranges image signals from the first line of a frame memory 64 to form a two-dimensional flaw detection image and, when arranges them up to a final line, again successively arranges new image signals from the first line to renew the two-dimensional flaw detection image. A flaw signal judging part 63 compares the flaw detection signal given from an A/D converter 5 with a threshold and, in the case it is equal to the threshold or more, the flaw signal judging part 63 judges a flaw primarily to apply a trigger to the image forming part 62 and the image forming part 62 designates a partial image containing the image signal when the trigger is applied and a CPU 61 applies the partial image to an image recording apparatus 7 to record the same. A flaw detection part 65 binarizes the partial image to specify an image thought to be a flaw and calculates data such as the length of the flaw, the width thereof or the distance thereof from an edge E on the basis of the specified image.