1. A three-dimensional ultrasonographic device, comprising: an ultrasonic transducer composed of a plurality of piezoelectric vibrators; a vibrator selecting part which causes an arbitrary one of the plural piezoelectric vibrators to generate an ultrasonic wave; a signal detecting circuit which selectively detects electrical signals that the plural piezoelectric vibrators generate when the piezoelectric vibrators receive echoes of the ultrasonic wave, which is generated by the piezoelectric vibrator selected by said vibrator selecting part to pass through an acoustic medium and to be reflected in an inspection object; a signal processing part which generates three-dimensional imaging data by aperture synthesizing from the electrical signals detected by said signal detecting circuit, in correspondence to meshes in a three-dimensional imaging area set in the inspection object; and a display processing part which has: a function of changing brightness or transparency with which respective meshes set in the three-dimensional area, according to values of the three-dimensional imaging data generated by the signal processing part, and a function of applying masking or image brightness correction to an image of an unnecessary portion of the three-dimensional imaging data, by multiplying the value of the three-dimensional imaging data by a value set according to a three-dimensional coordinate position (X, Y, Z).