To simply and accurately determine the position of an edge, the depth or the height of a flaw including a surface crack, a corroded portion near the surface layer of a thick specimen, and a minute damage.An ultrasonic flaw detection method causes an ultrasonic wave 16 to be incident on a specimen 6 from an oblique direction, detects a diffracted wave 17 generated at an edge of a flaw 20 in the specimen 6 above the flaw, and determines the position of the edge of the flaw 20 from a front surface 13 of the specimen 6 using a triangular method from an entire beam path Wt of a component 18, which directly propagates above the flaw 20, of the diffracted wave, the entire beam path passing through the flaw from the position of incidence of the component 18, and an interval S between the position of incidence and the position of detection of the ultrasonic wave or from the difference (tt−ts) between the propagation time of a surface wave 15, to which the above relations reflect, to reception and the propagation time of the diffracted wave 18, which directly propagates above the flaw, to reception.