1. A three-dimensional ultrasound imaging apparatus, comprising: a transducer array for transmitting ultrasound signals to a target object and receiving echo signals reflected from the target object, wherein the transducer array is comprised of a plurality of transducer elements; means for receive-focusing the echo signals from the transducer array to generate a plurality of consecutive two-dimensional ultrasound image frames; means for converting the consecutive two-dimensional ultrasound image frames into a three-dimensional ultrasound image by aligning the consecutive two-dimensional ultrasound image frames with no offset to each other in a three-dimensional space and interpolating a distance between two adjacent frames among the consecutive two-dimensional frames, to thereby make an inter-frame distance uniform; and a display for displaying the three-dimensional ultrasound image.