An ultrasonic flaw detector comprising: an ultrasonic probe including a plurality of piezoelectric elements each of which issues an ultrasonic wave (105; 105B) to an object of interest and detects its reflected ultrasonic wave signal occurring from the object; a pulser for supplying a driving signal to each piezoelectric element of the ultrasonic probe; a receiver for receiving a reception signal from each piezoelectric element of the ultrasonic probe; a delay time control section for assigning a different delay time to the driving signal and the reception signal for each of the piezoelectric elements; a data recording section for recording waveforms of the reflected ultrasonic wave signals received by the ultrasonic probe; an image processing computer for generating three-dimensional flaw detection data (201; 301) from the waveforms of the reflected waves recorded in the data recording section, wherein the computer is configured to acquire a reference reflected ultrasonic wave signal from a reference object to be compared with the object of interest, wherein the reflected ultrasonic wave signal from the object of interest having the same material and shape as the reference object is acquired under the same condition as the reference object, correct the reflected ultrasonic wave signal acquired from the object of interest based on the reference reflected ultrasonic wave signal of the reference object, and allow the three-dimensional display section to display the three-di-mensional flaw detection data (201; 301) generated from a reflected ultrasonic wave signal resulting from a difference between the reference object and the object of interest, characterised in that the ultrasonic flaw detector further comprises a three-dimensional display section for displaying the three-dimensional flaw detection data (201; 301) superimposed on three-dimensional shape data (202; 302) of the object, and the computer is configured to set, in the reflected ultrasonic wave signal from the object of interest, signal values at the same position as an extremum of the reference reflected ultrasonic wave signal and in the same area as a specific before-and-after area of the extremum so as to be sufficiently smaller than a maximum value of the reflected ultrasonic wave signal resulting from the difference between the reference object and the object of interest. an ultrasonic probe including a plurality of piezoelectric elements each of which issues an ultrasonic wave (105; 105B) to an object of interest and detects its reflected ultrasonic wave signal occurring from the object; a pulser for supplying a driving signal to each piezoelectric element of the ultrasonic probe; a receiver for receiving a reception signal from each piezoelectric element of the ultrasonic probe; a delay time control section for assigning a different delay time to the driving signal and the reception signal for each of the piezoelectric elements; a data recording section for recording waveforms of the reflected ultrasonic wave signals received by the ultrasonic probe; an image processing computer for generating three-dimensional flaw detection data (201; 301) from the waveforms of the reflected waves recorded in the data recording section, wherein the computer is configured to acquire a reference reflected ultrasonic wave signal from a reference object to be compared with the object of interest, wherein the reflected ultrasonic wave signal from the object of interest having the same material and shape as the reference object is acquired under the same condition as the reference object, correct the reflected ultrasonic wave signal acquired from the object of interest based on the reference reflected ultrasonic wave signal of the reference object, and allow the three-dimensional display section to display the three-di-mensional flaw detection data (201; 301) generated from a reflected ultrasonic wave signal resulting from a difference between the reference object and the object of interest,