1. A reference image display method comprising: generating an ultrasonic image of an object based on reflection echo signals output from an ultrasonic probe; extracting image data of a tomographic plane corresponding to a scan plane of the ultrasonic probe from pre-obtained volume image data of the object, in accordance with an association relationship between a coordinate system of the volume image data and a coordinate system of the ultrasonic image to reconstruct a reference image; displaying the ultrasonic image and the reference image on the same screen; freezing the reference image in accordance with an input instruction; performing an instruction for adjustment when the frozen reference image and the ultrasonic image corresponding to movement of the ultrasonic probe are displayed on the same cross section; correcting the association relationship between the coordinate system of the ultrasonic image and the coordinate system of the volume image data in accordance with the instruction for adjustment, based on a scan-plane coordinate of the ultrasonic probe during the adjustment and a scan-plane coordinate of the ultrasonic probe during the freezing.