1. An ultrasonic diagnosis apparatus comprising: a display, and an ultrasonic diagnostic system including a processor and a memory, where the ultrasonic diagnostic system operates to: generate an ultrasonic image of a scan-plane, from reflection echo signals output from an ultrasonic probe; store volume image data pre-obtained by a diagnostic imaging apparatus, in the memory; detect a three-dimensional position of a scan-plane coordinate by a position sensor, in accordance with a scan-plane coordinate system which is associated in advance with a volume image data coordinate system; extract reference image data corresponding to the ultrasonic image of the scan-plane from the volume image data, in accordance with an association of the scan-plane coordinate system and the volume image data coordinate system, and use the reference image data to reconstruct a reference image; display the ultrasonic image and the reference image on the display, respectively; freeze and maintain the freeze of the reference image responsive to a freeze instruction, to maintain a frozen reference image while change of the ultrasonic image is conducted responsive to movement of the ultrasonic probe; display the frozen reference image and a changing ultrasonic image corresponding to movement of the ultrasonic probe on the display, while the frozen reference image and the changing ultrasonic image are compared with each other; respond to an adjustment instruction when the frozen reference image and the changing ultrasonic image are found to display a same cross section, by determining a coordinate difference between the scan-plane coordinate system and the volume image data coordinate system, and correcting an association relationship between the scan-plane coordinate system of the ultrasonic image and the volume image data coordinate system in accordance with the adjustment instruction, based on the scan-plane coordinate of the ultrasonic probe at the adjustment instruction and the scan-plane coordinate of the ultrasonic probe at the freeze instruction.