An ultrasonic flaw detector for displaying only the shape echo and the defect echo needed for flaw evaluation, 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; and a three-dimensional display section for displaying the three-dimensional flaw detection data (201; 301) superimposed on three-dimensional shape data (202; 302), characterised in that the computer is configured to hide the three-dimensional flaw detection data (201; 301) and other shape echoes resulting from multi-reflection, the three-dimensional flaw detection data (201, 301) consisting of the defect echo and the shape echo resulting from normal reflection, included in a three-dimensional area by cutting off a two-dimensional area in a depth direction of the three-dimensional shape of the object along the viewing direction, wherein the two-dimensional area is specified by two points on the three-dimensional display section, and the diagonal of the two-dimensional area corresponds to a straight line connecting the two points, the computer is further configured to determine whether the hidden area contains voxels configuring the 3D flaw detection area (201, 301) by checking the relation between each voxel coordinate value and a spatial area that specifies the hidden area, and the computer is further configured to redraw the three-dimensional flaw detection data (201, 301) by redrawing only voxels including three-dimensional flaw detection data (201, 301). 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; and a three-dimensional display section for displaying the three-dimensional flaw detection data (201; 301) superimposed on three-dimensional shape data (202; 302), characterised in that the computer is configured to hide the three-dimensional flaw detection data (201; 301) and other shape echoes resulting from multi-reflection, the three-dimensional flaw detection data (201, 301) consisting of the defect echo and the shape echo resulting from normal reflection, included in a three-dimensional area by cutting off a two-dimensional area in a depth direction of the three-dimensional shape of the object along the viewing direction, wherein the two-dimensional area is specified by two points on the three-dimensional display section, and the diagonal of the two-dimensional area corresponds to a straight line connecting the two points, the computer is further configured to determine whether the hidden area contains voxels configuring the 3D flaw detection area (201, 301) by checking the relation between each voxel coordinate value and a spatial area that specifies the hidden area, and the computer is further configured to redraw the three-dimensional flaw detection data (201, 301) by redrawing only voxels including three-dimensional flaw detection data (201, 301).