1. An ultrasound diagnostic device comprising, an ultrasound probe which emits an ultrasound wave and receives a reflected wave of the ultrasound wave, a first position detector which detects a position of the ultrasound probe, an ultrasound image generator which generates an ultrasound image by using a reflected echo signal based on the reflected wave, a reference image generator which generates a reference image of an arbitrary cross section by using three-dimensional volume data of a test object and displays an ultrasound probe mark indicating the position of the ultrasound probe in a superimposed manner on the reference image, and an image processor which displays the ultrasound image and the reference image on a display.