A three-dimensional ultrasonic imaging apparatus, comprising: an ultrasonic transducer in which a plurality of piezoelectric vibrators are disposed in a matrix or an array; a driving element selecting portion connected to the piezoelectric vibrators and configured to select a piezoelectric vibrator to emit an ultrasonic wave from the ultrasonic transducer from among the plurality of the piezoelectric vibrators; a signal detecting circuit configured to cause ultrasonic waves emitted by the piezoelectric vibrator selected by the driving element selecting portion to enter an object to be inspected via an acoustic wave propagating medium, to receive an echo reflected by the object to be inspected and thereby to detect an electric signal corresponding to the reflected echo; a signal processing portion configured to generate three-dimensional imaging data by subjecting the electric signal corresponding to the detected reflected echo to parallel arithmetic processing while causing an inside of the object to be inspected to correspond to mesh elements in a three-dimensional imaging region partitioned in advance; and a display processing device configured to receive the three-dimensional imaging data from the signal processing portion, to correct image luminance distribution of the three-dimensional imaging data so as to flatten an imaging intensity distribution in a plane direction of a surface of the object to be inspected and then to display a corrected result, characterised in that the display processing device also has a flattening function of correcting the image luminance distribution of the three-dimensional imaging data so as to eliminate fluctuations of the image luminance distribution, such fluctuations causing a surface image of the three-dimensional imaging data to be strong at its center and weak at its periphery, and such fluctuations being caused by unevenness of irradiation of the ultrasonic wave. an ultrasonic transducer in which a plurality of piezoelectric vibrators are disposed in a matrix or an array; a driving element selecting portion connected to the piezoelectric vibrators and configured to select a piezoelectric vibrator to emit an ultrasonic wave from the ultrasonic transducer from among the plurality of the piezoelectric vibrators; a signal detecting circuit configured to cause ultrasonic waves emitted by the piezoelectric vibrator selected by the driving element selecting portion to enter an object to be inspected via an acoustic wave propagating medium, to receive an echo reflected by the object to be inspected and thereby to detect an electric signal corresponding to the reflected echo; a signal processing portion configured to generate three-dimensional imaging data by subjecting the electric signal corresponding to the detected reflected echo to parallel arithmetic processing while causing an inside of the object to be inspected to correspond to mesh elements in a three-dimensional imaging region partitioned in advance; and a display processing device configured to receive the three-dimensional imaging data from the signal processing portion, to correct image luminance distribution of the three-dimensional imaging data so as to flatten an imaging intensity distribution in a plane direction of a surface of the object to be inspected and then to display a corrected result, characterised in that the display processing device also has a flattening function of correcting the image luminance distribution of the three-dimensional imaging data so as to eliminate fluctuations of the image luminance distribution, such fluctuations causing a surface image of the three-dimensional imaging data to be strong at its center and weak at its periphery, and such fluctuations being caused by unevenness of irradiation of the ultrasonic wave.