To provide an ultrasonic flaw detection method and an ultrasonic flaw detector for reliably detecting even minute defects in a body to be inspected, when detecting defects existing inside a body to be inspected by utilizing the attenuation of ultrasonic waves transmitted through the body to be inspected.

In the ultrasonic flaw detection method, defects in the body to be inspected are detected from ultrasonic echo intensity 13 in which ultrasonic waves discharged from a transmission probe 2a are transmitted through the body 1 to be inspected and reach a reception probe 2b, or defects in the body to be inspected are detected from the ultrasonic echo intensity 13 in which ultrasonic waves discharged from a probe 2 are transmitted through the body 1 to be inspected and are reflected by a reflector plate 3, are transmitted through the body 1 to be inspected again, and reach the probe 2. The ultrasonic waves are reflected from one end face of the body 1 to be inspected, are reflected from the other end face, and then the attenuation of intensity in transmission echoes (multiple reflection transmission echo 11) reaching the probe 2 is measured to detect defects in the body to be inspected.