A three-dimensional reference image is displayed along with an ultrasound image (tomogram). The three-dimensional reference image has a first existing line representing a first puncture needle and a second prospective line indicating the prospective puncture path of a second puncture needle. The position and orientation of the probe are adjusted by the user with reference to the three-dimensional reference image so that the second prospective line has an appropriate position relative to the first existing line. After the adjustment, the second puncture needle is inserted into the body using a puncture adaptor provided on the probe. A second existing line is displayed in the three-dimensional reference image when the insertion of the second puncture needle is completed.