A three-dimensional ultrasonic inspection apparatus includes: a sensing device for ultrasonic inspection including an ultrasonic sensor as a transducer, in which a plurality of piezoelectric vibrators are disposed in a matrix or an array; a drive element selecting unit for selecting and driving a piezoelectric vibrator for producing an ultrasonic wave among the ultrasonic transducer; a signal detecting circuit for detecting the electric signal of the reflected echo by receiving the reflected echo from the joined area; a signal processing unit for subjecting the electric signal to signal processing and generating three-dimensional imaging data by causing the electric signals to correspond to mesh elements in a three-dimensional imaging region of the object to be inspected; and a display processing device for displaying the detection results and three-dimensional image data from the signal processing unit, while detecting the size and the position of the molten-solidified portion, and the size and the position of the weld defect of the joined area, from the intensity distribution of the three-dimensional imaging data generated by the signal processing unit.