1. An ultrasound diagnostic apparatus comprising: a first probe; a second probe; a first transmitter/receiver configured to scan the inside of an object to be examined with a first ultrasound beam through said first probe to acquire first ultrasound image data associated with the object; a second transmitter/receiver configured to scan the inside of the object with a second ultrasound beam through said second probe to acquire second ultrasound image data associated with the object; a position detector configured to detect a position of said first probe relative to said second probe; an image processor configured to synthesize the second ultrasound image data with the first ultrasound image data on the basis of the detected position to generate a synthetic image data; and a display configured to display an image on the basis of the synthetic image data.