1. An ultrasonic flaw detector comprising: an ultrasonic probe including a plurality of piezoelectric elements each of which issues an ultrasonic wave 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 from the waveforms of the reflected waves recorded in the data recording section; and a three-dimensional display section for displaying the three-dimensional flaw detection data superimposed on three-dimensional shape data of the object, 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, correct the reflected ultrasonic wave signal acquired from the object of interest having the same material and shape as the reference object, based on the reference reflected ultrasonic wave signal of the reference object, and allow the three-dimensional display section to display the three-dimensional flaw detection data generated from a reflected ultrasonic wave signal resulting from a difference between the reference object and the object of interest.