1. An ultrasonic flaw detector comprising: an ultrasonic probe including a plurality of piezoelectric elements which issues at least one ultrasonic wave to an object of interest and detects corresponding reflected ultrasonic wave signals occurring from the object of interest; a pulser for supplying a driving signal to each piezoelectric element of the ultrasonic probe to issue the ultrasonic wave from the piezoelectric elements; a receiver for receiving a reception signal to be the reflected ultrasonic wave signals 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 data of the reflected ultrasonic wave signals received by the ultrasonic probe; an image processing computer for acquiring reference reflected ultrasonic wave signals from a reference object to be compared with the object of interest, correcting the reflected ultrasonic wave signals acquired from the object of interest having the same material and shape as the reference object, based on the reference reflected ultrasonic wave signals of the reference object, generating three-dimensional flaw detection data based on a difference between the reference reflected ultrasonic signals as to the reference object and the reflected ultrasonic wave signals as to the object of interest, and displaying on a three-dimensional display section the three-dimensional flaw detection data superimposed on previously acquired three-dimensional shape data of the object of interest.