To provide a plate wave ultrasonic flaw detection method reducing the noise level contained in a flaw detection signal and preventing the erroneous detection of a flaw.

The memory 62 of a computer 6 successively stores respective flaw detection signals to which the flaw detection signal related to an upstream tire probe 11 is imparted. An operation part 63 calculates the number of times of the pulse signals outputted until the part subjected to flaw detection by the upstream tire probe 11 of the material S to be inspected is subjected to flaw detection by the downstream tire probe 11 on the basis of the transfer speed of a material S to be flaw-detected taken in from a speed detector 9, the distance between tire probes 11, 11 and the cycle of a pulse signal. An operation part 63 takes in the flaw detection signal related to the tire probe 11 arranged on the downstream side to read the flaw detection signal stored previously by the calculated number of times from the memory 62 to reverse one flaw detection signal and subsequently superposes both signals one upon another to calculate an average. A flaw detection part 64 compares the intensity of the signal subjected to averaging processing with a threshold and judges that there is a flaw in the case it is equal to the threshold or more to calculate the data related to the flaw.