1. An image processing device comprising: an image obtaining unit for obtaining a three-dimensional moving image showing a body part of a patient and an ultrasonic moving image showing the body part, the body part making a predetermined periodic motion; a characteristic part extracting unit for extracting, from a plurality of frame images forming the obtained three-dimensional moving image and ultrasonic moving image, a predetermined characteristic part having a shape that changes in response to the periodic motion; a phase obtaining unit for obtaining phases of the periodic motion captured in the three-dimensional moving image and the ultrasonic moving image, wherein, for at least one of the three-dimensional moving image and the ultrasonic moving image, the phases are obtained based on the shape of the extracted characteristic part; an associating unit for associating, for each phase, a position of the characteristic part in a frame image forming the three-dimensional moving image corresponding to the phase with a position of the characteristic part in a frame image forming the ultrasonic moving image corresponding to the phase based on the extracted characteristic part and the obtained phases; an image generating unit for generating a comparison image of each of the three-dimensional moving image and the ultrasonic moving image by aligning, for each phase, a position of the characteristic part in a frame image forming the three-dimensional moving image corresponding to the phase with a position of the characteristic part in a frame image forming the ultrasonic moving image corresponding to the phase based on the associated positions of the characteristic part and the phases, the alignment is achieved by transforming the coordinate system of the three-dimensional moving image into the coordinate system of the ultrasonic moving image; and a display controlling unit for displaying the generated comparison images in a manner allowing comparison therebetween on a display device.