1. An ultrasonic image generating method for generating an ultrasonic image based on ultrasonic echo data obtained by transmitting ultrasonic waves to and receiving ultrasonic waves from an inspection object, the method comprising: a surface extraction step for obtaining, on a plurality of consecutive two-dimensional tomograms obtained in accordance with the ultrasonic echo data, positions where an average luminance at a constant thickness of an object on the two-dimensional tomograms is equal to or greater than a predetermined value as extraction points on a surface inside a body cavity, based on a luminance of the two-dimensional tomograms and the thickness of the object; an extraction point correction step for calculating expected extraction points based on a result of obtaining other extraction points adjacent to the extraction points and then correcting the extraction points using the expected extraction points; a reference position determining step for performing, using a computer, a processing for converting surface coordinate in accordance with the extraction points corrected by the extraction point correction step from polar coordinates to Cartesian coordinates, and then calculating a center position of the body cavity as a reference position on the two-dimensional tomograms based on coordinate values disposed clockwise at positions of 3, 6, 9, and 12 o′clock on the Cartesian coordinates; a correction step for obtaining, using a computer, a plurality of regular and consecutive two-dimensional tomograms by correcting irregularity of the reference position of each of the plurality of consecutive two-dimensional tomograms determined by the reference position determining step; and an ultrasonic image generating step for generating, using a computer, three-dimensional ultrasonic images based on the plurality of regular and consecutive two-dimensional tomograms corrected by the correction step.