Abstract: An ultrasonic flaw detecting apparatus for efficiently detecting a flaw existing in a deep portion of a multi-layer structure includes a first sensor arranged so as to transmit an ultrasonic wave into a different property layer surrounded by a medium having a different acoustic impedance in the multi-layer structure, a function generator for generating a pulse-shaped sine wave, a power amplifier for amplifying and supplying the pulse-shaped sine wave to the first sensor, a second sensor arranged so as to receive a boundary echo from the different property layer, a first amplifier for amplifying the boundary echo received by the second sensor, an intensity detector for judging the intensity of an amplified signal of the boundary echo, a third sensor arranged so as to receive an echo from a flaw inside a layer in a deep portion of the different property layer, a second amplifier for amplifying the flaw echo received by the third sensor, a recorder for recording the amplified flaw echo signal, and a control unit